Measurement of health &

disease

UNIVERSITY OF GONDAR
COLLAGE OF MEDICINE AND HEALTH SCIENCE
INSTITUTE OF PUBLIC HEALTH
DEPARTMENT OF EPIDEMIOLOGY AND
BIOSTATISTICS
Alemneh M. (BSc, MPH)

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Course objective

• At the end of this course students will be able to

apply public health methods for measurement of
health and disease at population level

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Course contents
Chapter one: Introduction to public health

Chapter two: Epidemiological concepts of

disease causation

Chapter three:Measuremment of disease

and death

Chapter four: Epidemiological studies

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Chapter Five: measurements of
association

Chapter six: Public health

surveillance

Chapter seven: Outbreak

investigation and management
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Assessment Methods

 Continuous Assessment: Class

Participation, Assignments , Quizzes,
Tests/mid-examination, and etc – 50%

 Final Written Examination – 50%

 

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 Chapter One
Introduction to Public health
Definitions of terms:
Health is a difficult concept to define.
WHO in 1948 defined health as “A state of complete
physical, mental and social well being and not merely
the absence of disease or infirmity.”

Public health - a science and an art of

preventing disease, prolonging life, and promoting
health and efficiency through organized
community effort for sanitation, control of
communicable disease, health education, etc.

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 Clinical versus community medicine:

Clinical medicine is concerned with

diagnosing and treating diseases in
individual patients, while community
medicine is concerned with
diagnosing the health problems of a
community, and with planning and
managing community health
services.

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 Methods of Community
Diagnosis:
 Discussion with community leaders
and health workers
 Survey of available health records
 Field survey.
 Compilation and analysis of the data.

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 It is impossible to address all the identified
problems at the same time because of
resource scarcity.
 Therefore the problems should be put in
the order of priority using a set criterion.

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 Criteria for priority setting
 Magnitude (amount or frequency) of the
problem
 Severity (to what extent is the problem
disabling, fatal)
 Feasibility (availability of financial and
material resource, effective control
method)
 Community concern (whether it is a felt
problem of the community)
 Government concern (policy support,
political commitment)
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 Health p1 2 3 4
Magnitude
Severity
Feasibility
Community
concern
Gov’t
concern
Total

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EPIDEMIOLOGY

It can be defined as the study of the

frequency,
distribution and
determinants of diseases and health
related states or events in specified
human populations and the application
of this study to the promotion of health
and to the prevention and control of
major health problems

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 Components of the
definition
 “Population” the focus of epidemiology is
mainly on the population rather than
individuals.
 “Frequency” shows epidemiology is
mainly a quantitative science.
Epidemiology is concerned with the frequency
of diseases and other health related
conditions.
Frequency of diseases and deaths are
measured by morbidity rates and mortality
rates.
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 “Health related conditions” are
conditions which directly or indirectly
affect or influence health. These may be
injuries, vital events, health related
behaviors, social factors, economic
factors etc.

 “Distribution” Refers to the distribution

of diseases in time, place and person.

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 The part of epidemiology concerned with
the frequency and distribution of diseases
by time, person and place is named
Descriptive Epidemiology.
.

 It asks the questions: How many?

Where? When? What?

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Determinants” are factors which
determine whether or not a person will get a
disease.

The part of epidemiology dealing with the

causes and determinants of diseases is
Analytical Epidemiology.

It asks the questions: how? Why?

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 Scope of Epidemiology
 Originally, epidemiology was concerned with
epidemics of communicable diseases
and epidemic investigations.
 Later it was extended to endemic
communicable diseases and non-
communicable diseases

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At present epidemiologic methods are
being applied to:

Infectious and non infectious diseases

Injuries and accidents
Nutritional deficiencies
Mental disorders
 Maternal and child health
Congenital anomalies
Cancer
Occupational health
Environmental health
Health behaviors
Violence etc.
all disease conditions and other health related events.

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 Purpose/Use of
Epidemiology
The ultimate purpose of Epidemiology is
prevention and control of disease, in an effort
to improve the health status of populations.
This is realized through:
1. Elucidation of the natural history of disease

2. Description of the health status of the population

3. Establishing the determinants/causation of disease

4. Evaluation of intervention

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 Fundamental Assumptions in
Epidemiology
There are two basic assumptions in
epidemiology.
1. Non random distribution of diseases
i.e. the distribution of disease in human
population is not random or by chance
2. Human diseases have causal and
preventive factors that can be
identified through systematic
investigations of different populations.

Epidemiology uses systematic approach to

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study the differences in disease
 Risk Factors:
• Any factor associated with an increased or
decreased occurrence of disease.
• A factor associated with an increased
occurrence of a disease is risk factor for the
exposed group; and a factor associated
with a decreased occurrence of a disease is
a risk factor for the non exposed group.

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Risk factors could be:
 Factors related to the agent: Strain difference
 Factors related to the human host: Lack of
specific immunity.
 Factors related to the environment:
Overcrowding, Lack of ventilation
Risk factors may further be classified as:
• Factors susceptible to change: smoking
habit, alcohol drinking habit
• Factors not amenable to change: age, sex,
family history

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 In order to be able to prevent disease, it is
vital to identify factors that can be changed.
 For some diseases, the specific causes are
not known. In such cases it is very
important to identify risk factors, especially
those that can be changed and act on them.
 Epidemiology is mainly interested in those
risk factors that are amenable to change as
its ultimate purpose is to prevent and
control disease and promote the health of
the population.

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 Chapter Two
Epidemiological concept of disease
causation

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 HOW THE DISEASE IS CAUSED?

 20th century theories

1. Supernatural theory of disease

2. Ecological theory

3. Germ theory

4. Multifactorial causation theory

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25
25
 1. SUPERNATURAL THEORY OF DISEASE
• At least 10% of the people in developed countries
and 30% in developing countries still believe in
supernatural origin
• Even today superstitions are becoming major
obstacles in disease control
• Most of the literates view that disease is the result of
microbes
• Most of the uneducated people (90%) believe that
disease is due to bad physical environment

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 2. ECOLOGICAL THEORY
• Around 463 BC, hippocrates is the first
epidemiologist who advised to search the
environment for the cause of the disease.
• Mckeown has pointed out, improved health
owes less to advances in medical science
than to the operation of natural ecological
laws
• He rightly advised to search air, water and
places for the cause of a disease
• In ecological approach an agent is
considered necessary but not sufficient
cause
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of a disease.
27
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 3. GERM THEORY
Germ theory: Microbes
(germs) were found to be
the cause for many known
diseases.

Pasteur, Henle and Koch

were the strong proponents
of microbial theory after
ROBERT KOCH
they discovered the micro-
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28 patients’
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 HENLE-KOCH’S POSTULATES
 Koch's Postulate : a set of rules for
determination of causation, states that:-

1. The organism must be present in every case.

2. The organism must be isolated and grown in

culture.

3. The organism must, when inoculated into a

susceptible animal, cause the specific disease.

4. The organism must then be recovered from the

animal.
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The requirement that more than one factor be
present for
disease to develop is referred to as multiple
causation or
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multifactorial etiology 30 30
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31 31
 DISEASE CAUSATION MODELS

• How do diseases develop?

• Epidemiology helps researchers visualize
disease and injury etiology through
models. Best known disease causation
models are
1. Epidemiological triangle
2. Web of causation
3. Wheel model

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 1. The epidemiologic triangle

Agent
 

Host
Environment

• The most familiar disease model, the epidemiologic triad

(triangle), depicts a relationship among three key factors in the
occurrence of disease or injury.

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 Epidemiological triad model con’t…
• An agent is a factor whose presence or
absence, excess or deficit is necessary for a
particular disease or injury to occur.
• General classes of disease agents include;
1. Chemicals such as benzene, oxygen, and
asbestos;
2. Microorganisms such as bacteria, viruses, fungi,
and protozoa; and
3. Physical energy sources such as electricity and
radiation.
• Many diseases and injuries have multiple agents.

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• The environment includes all external
factors, other than the agent, that can
influence health. These factors are further
categorized according to whether they
belong in the social, physical, or
biological environments.
• The social environment including
availability of medical care and health
insurance; cultural “dos” and “don’ts”
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• The physical environment:- climatic
conditions, pollution…
• The biological environment:- vectors,
humans and plants and animals acting as
a reservoir
• The host is the actual or potential
recipient or victim of disease or injury.
Although the agent and environment
combine to “cause” the illness or injury if
the host is not susceptible disease will not
occur.
• According to this model Disease and injury
occur only when the interaction between
the three factors is altered.
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 2. The web of causation model

• It was developed especially to enhance understanding of

chronic disease, such as cardiovascular disease. However, it
can also be applied to the study of injury and communicable
diseases
Stress diet

Hormones physical
inactivity

Smoking obesity
heredity
Blood clotting hardening of arteries
Heart disease stroke hypertension

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 3. The Wheel model The wheel consists of a
hub (the host or human), which has genetic makeup
as its core. Surrounding the host is the environment.
The size of each part varies according to the type of
disease shown.
Social
Physical environment
environment
Host
(man)
vc
Genetic
core

Biologic
environment

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 Natural History of Disease
and Levels of Prevention

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Natural history of disease
 Is the course of a disease over time
without any treatment or
intervention.

 Each disease has its own life history,

and thus, any general formulation of
this process is arbitrary.

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 There are four stages in the natural
history of a disease.
These are:
Stage of susceptibility/exposure
Stage of pre-symptomatic (sub-clinical)
disease
Stage of clinical disease and
Stage of disability or death

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1. Stage of susceptibility
Disease has not yet developed, but there
are factors that favor its occurrence

Examples:
 A person practicing casual and
unprotected sex has a high risk of
getting HIV infection.
 An unvaccinated child is susceptible to
measles.
 High cholesterol level increases the risk
of coronary heart disease.

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 2.Stage of Pre-symptomatic
(sub-clinical) disease
In this stage there is no manifestation of
the disease but pathogenic changes have
started to occur.
There are no detectable signs or symptoms.
The disease can only be detected through
special tests.
Examples:
 Detection of antibodies against HIV in an
apparently healthy person.
 Ova of intestinal parasite in the stool of
apparently healthy children.
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 3. The Clinical stage
 By this stage the person has developed signs and
symptoms of the disease.
 The clinical stage of different diseases differs in
duration, severity and outcome.
 The outcomes of this stage may be recovery,
disability or death.
Examples:
 Common cold has a short and mild clinical stage
and almost everyone recovers quickly.
 Polio has a severe clinical stage and many patients
develop paralysis becoming disabled for the rest
of their lives.
 Rabies has a relatively short but severe clinical
stage and almost always results in death.
 HIV/ AIDS has a relatively longer clinical stage and
eventually results in death.
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 4. Stage of disability or death
 The disease has occurred and left
damage to the body that limits the
activity of the victim(disability) or has
ended with the death of the victim

Examples:
 Trachoma may cause blindness
 Meningitis may result in blindness,
deafness or death.

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 Healthy person

Sub clinical disease

Recovery
Clinical disease

Recovery Death
Disability

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 Disease Prevention
 The major purpose in investigating the
epidemiology of diseases is to learn how to
prevent and control them.
 Disease prevention means to interrupt or
slow the progression of disease.

 Epidemiology plays a central role in disease

prevention by identifying those modifiable
causes.

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Levels of disease prevention

1. Primary
health promotion
Prevention of exposure
Prevention of disease
2. Secondary prevention
Screening
Early detection and treatment
3. Tertiary prevention
Rehabilitation

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 1. Primary prevention
 Is aimed at preventing healthy people from
becoming sick.

 The main objectives of primary prevention are

promoting health, preventing exposure and
preventing disease.

 Primary prevention keeps the disease process

from becoming established by eliminating
causes of disease or increasing resistance to
disease.
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 1.1 - Health promotion
 Consists of general non-specific
interventions that enhance health and the
body's ability to resist disease.
 Improvement of socioeconomic status,
provision of adequate food, housing,
clothing, and education are good examples
of health promotion.

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 1.2 - Prevention of
exposure
is the avoidance of factors which may
cause disease if an individual is
exposed to them.

 Examples can be provision of safe and

adequate water, proper excreta
disposal, and vector control.

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 1.3 - Prevention of
disease
 Is the prevention of disease development
after the individual has become exposed
to the disease causing factors.

 The timing is between exposure and

biological onset.

 Post-exposure prophylaxis for HIV

exposed individuals

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2. Secondary prevention
 Detecting people who already have the
disease as early as possible and treat
them.
 It is carried out after the biological onset
of the disease, but before permanent
damage sets in.
 The objective of secondary prevention is
to stop or slow the progression of
disease and to prevent or limit
permanent damage.
Examples:
 Prevention of blindness from Trachoma
 Early detection and treatment of breast
cancer to prevent its progression to the
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invasive stage
 3. Tertiary prevention
 Is targeted towards people with chronic
diseases and disabilities that cannot be
cured.
 Tertiary prevention is needed in some
diseases because primary and secondary
prevention have failed, and in others
because primary and secondary prevention
are not effective.

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 Tertiary prevention
cont’d
It has two objectives:
 Treatment to prevent further disability or
death and
 To limit the physical, psychological,
social, and financial impact of disability,
thereby improving the quality of life.
 This can be done through rehabilitation,
which is the retaining of the remaining
functions for maximal effectiveness.

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Level of prevention Point of intervention
Natural history of disease
Primary Health promotion
Prevention of exposure

Prevention of disease exposure

Secondary  
Early detection &treatment Biological
=screening onset incubation
(prevention of clinical onset )
 
period

 
Clinical
onset
Early treatment
(prevention of permanent damage
)

Tertiary Limitation of disability Permanent

  damage

Rehabilitation (prevention of
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 Exercise -1
for each of the following intervention , indicate what
level of prevention is involved

1. Special education for the handicapped children.

2. Treatment of people with a diagnosis of active TB.
3. Screening donated blood to exclude
contaminating with causation agent of HBV.
4. Promoting awareness in high school students of
self-protective measure against HIV/AIDS.
5. Counseling AIDS patients and their families with
the objective of helping them find ways of
relieving their mental stress and financial burden
imposed by the illness .
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 Exercise -2

For each of the following components of

ANC indicate what level of prevention
involved for the child and for the mother.
1. VDRL testing for syphilis.
2. Immunization against tetanus.
3. Monitoring of blood pressure.
4. Providing chemoprophylaxis for malaria.
5. Early treatment of urinary tract
infection.

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 Thank you!

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 CHAPTER THREE
Measurement of Disease and
Death

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 Measurements of Disease

In Epidemiology there are two ways of measuring

morbidity
1. Incidence rate
2. Prevalence rates

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Incidence Rate

 The incidence of a disease is defined as

the number of new cases of a disease that
occur during a specified period of time in a
population at risk for developing the
disease.

• Because incidence is a measure of new

events (i.e. transition from a non-diseased
to a diseased state), It is a measure of
risk.
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Types of incidence
1. Cumulative Incidence (CI)

 An incidence rate that is calculated from a

population that is more or less stable (little
fluctuation over the interval considered), by
taking the population at the beginning of the
time period as denominator.

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CI = Number of new cases of a disease
during a given period of time
X 1000
Total population at risk

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2. Incidence Density
 An incidence rate whose denominator is calculated
using person-time units.

 Similar to other measure of incidence, the

numerator of the incidence density is the number
of new cases in the population.

 The denominator, however, is the sum of each

individual’s time at risk or the sum of the time that
each person remained under observation, i.e.,
person – time denominator.
 This is particularly when one is studying a group
whose members are observed for different lengths
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 ID =Number of new cases during
a x10n
given period
Time each person was observed, totaled for
all

 1 individual is followed for 1 yr = 1pyr

 1 individual is followed for 4 years = 4pyr
 4 individuals followed for 1 = 4pyr
 1 individual is followed for 1 yr = 12
pmonth=365pday
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 1individual is followed for
Lab tech rotation to hematology section
Total number of students
Stay in hematology lab person year
contrib. 60
20 students for 3 months 20X1/4
20 students for 6 months 20X1/2
20 students for 1 year 20X1
35 person yr
If 10 students develop hepatitis=(10/35)X100
= 28.57 new infections /100 person
years of observation

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2. Prevalence rate
 Prevalence rate measures the number of
people in a population who have a disease
at a given time.

 It includes both new and old cases.

There are two types of prevalence rates.

1. Period Prevalence rate
2. Point Prevalence rate

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 1. Period prevalence rate
 Period Prevalence rate measures the
proportion of a population that is affected with
a certain condition during a specified period of
time.

PPR = # people with a condition during a specific

period of time X10n average population

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 Point Prevalence rate
 Measures the proportion of a population
with a certain condition at a given point in
time.
 This is not a true rate; rather it is a simple
proportion.

 PtPR=All persons with a specific condition at

a point in time *1000
Total population

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 Exercise 1
Each horizontal line in the next figure
represents one of 10 persons who had
giardiasis (a parasitic intestinal
infection which doesn’t confer
immunity against subsequent
infections) in a population of 100.
The date of onset and duration of
each episode indicated by a line of
X’s .the vertical lines indicate specific
dates.
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a. Population at risk for giardiasis on Jan 1, 1990
b. Incidence rate of giardiasis in 1990
c. Point prevalence rate of giardiasis on Jan 1 1990
d. The period prevalence of giardiasis in 1990
e. The point prevalence of giardiasis on July1, 1990

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• A=98
• B=6/98*1000=6.1per1000
• C=20per 1000
• D=80per 1000
• E=40per 1000

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Relationship between incidence and point prevalence

 Since point prevalence rate includes both

new and pre-existing cases, it is directly
related to the incidence rate.
 Point prevalence rate is directly
proportional both to the incidence rate and
to the average duration of the disease.

 Point Prevalence rate = IR x D

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11/14/19 76
 Factors influencing prevalence
rate
a) severity of illness - for highly fatal
(lethal) diseases the prevalence rate is
low.
b) duration of illness - short duration of
disease leads to low prevalence rates.
c) the number of new cases - the higher
the number of new cases the higher will
be the prevalence.

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 Prevalence can be increased by

 Longer duration of the disease

 Prolongation of life of patients without cure
 Increase in new cases (increase in
incidence)
 In-migration of cases
 Out-migration of healthy people
 Improved diagnostic facilities and better
reporting

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 Prevalence can be
decreased by
 Shorter duration of disease
 High case - fatality rate
 Decrease in new cases
 In-migration of healthy people
 Out-migration of cases
 Improved cure rate of cases

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 Prevalence rate is not a helpful measure to
provide strong evidence of causation and
risk of development of a disease.

Prevalence rates are important

particularly for
Chronic disease studies
Planning health facilities and manpower
Monitoring disease control programs
Tracking changes in disease patterns over
time

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Incidence rate is important as
A fundamental tool for etiologic studies
of acute and chronic diseases
A direct measure of risk

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 Limitations of prevalence
studies
 Prevalence studies favor inclusion of
chronic over acute cases.
 Disease status and attribute are measured
at the same time; hence, temporal
relations cannot be established.

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Measurements of Death

Mortality rates and ratios

• Mortality rates and ratios measure the
occurrence of deaths in a population using
different ways.
• Below are given some formulas for the
commonly used mortality rates and ratios.

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 Crude Death rate (CDR)
 CDR= Total no. of deaths reported X
1000
during a given time interval
Estimated mid interval
population

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 Age- specific mortality rate

= No. of deaths in a specific age group during a given

time X1000
Estimated mid interval population of sp. age group

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 Sex- specific mortality rate

= No. of deaths in a specific sex during a given

time X 1000
Estimated mid interval population of same sex

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 Cause- specific mortality rate

CSDR= No. of deaths from a specific

cause during a given time X 100,000
Estimated mid interval population

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 Proportionate mortality ratio

PMR= No. of deaths from a sp. cause during a given

time x 100
Total no. of deaths from all causes in the same
time

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 Case Fatality Rate
CFR= No. of deaths from a sp. disease during a given
time x 100
No. of cases of that disease during the same
time

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Perinatal Mortality Rate

PMR= No. of fetal deaths of 28 wks or more

gestation
Plus no. of infant deaths under 7 days
Still birth plus the no. of live births during the
same

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 Neonatal Mortality Rate

NMR= No. of deaths under 28 days of age

reported
during a given time x 1000
No. of live births in the same area& during the
same year

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 Infant mortality rate
IMR= No. of deaths under 1 yr during a given time
X 1000
No. of live births in the same area& during
the same yr

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 Child mortality rate
= No. of deaths of 1-4 yrs of age during a given
time X 1000
Average (mid-interval) population of same age at
same time

Under- five mortality rate

= No. of deaths of 0-5 yrs of age during a given
time X 1000
Average (mid-interval) population of the same
age
at same time

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 Maternal Mortality Ratio
MMR=Total no of female deaths due to complications of
pregnancy,
child birth & puerprium in an area in a year x
100,000
No. of live births in the same area & year

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 When calculating (using) mortality rates it is
important to understand their interpretations and
how they differ from each other.

 For example case fatality rate; proportionate

mortality ratio, and cause specific death rates are
often confused.

 They all have the same numerator, i.e. number

of deaths from a specified cause, occurring in a
specified population, over a specified period of
time.

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 The case fatality rate asks the question: “what
proportion of the people with the disease
die of the disease?”

 The proportionate mortality ratio asks the

question:” out of all the deaths occurring in
that area, what proportion are due to the
cause under study?”

 The cause specific death rate asks the question:

“out of the total population, what
proportion dies from a certain disease
within a specified period of time?”

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 SCREENING AND SCREENING
PROGRAM EVALUATION

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Definition
• Screening is the search for unrecognized disease or

defect by means of rapidly applied tests, examinations

or other procedures in apparently healthy individuals.

• A screening test is not intended to be diagnostic.

• Screening is an initial examination only, and positive

responders require a second, diagnostic examination

• When screening tests are applied to large, unselected

populations, the process is called mass screening.

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 Case finding (diagnosis)
• Occurs when clinicians search for diseases with

tests among their own patients who are

consulting them for symptoms/complaints

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 Objective of Screening Tests
• To lower morbidity and mortality of the
disease in a population.
• Screening provides access to the medical
care system which is not an actual goal of
screening, but is a benefit.

To reverse, halt, or slow the progression of a

disease to its severe form and to improve
quality of life
o Research: study on natural history of disease
o Selection of healthy individuals usually for
employment
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11/14/19 101
 Primary requirements for
screening
1. Pre-clinical disease left untreated typically
progresses to clinically-evident disease (e.g.
no spontaneous regression).
2. The disease should be serious (relates to cost
effectiveness, ethics, and prognosis).
3. Prevalence of pre-clinical disease should be
relatively high among those screened.
4. Treatment given before symptoms develop
should be more beneficial in terms of reducing
morbidity or mortality than that given after
they develop

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 An ideal screening test should be

1. Inexpensive,

2. Easy to administer,

3. impose minimal discomfort on the

patients,

4. Valid and reliable.

11/14/19 103
 Validity of a screening test

• How good is the screening test

compared with the confirmatory
diagnostic test?

• Validity of a test is the ability to

differentiate accurately between
those who have the disease and
those who do not.

11/14/19 104
 Sensitivity and Specificity of a screening test

A.Sensitivity - is the ability of a test to identify

correctly those who have the disease. The
test will actually classify a diseased person as
likely to have the condition.

B. Specificity - is the ability of a test to identify

correctly those who do not have the disease.
The test will actually classify a non-diseased
person as unlikely to have the condition.

11/14/19 105
Test Definitive diagnosis
resul Diseased Non Total
t diseased

Positive TP (a) FP (b) TP+FP

Negative FN (c) TN (d) TN+FN
Total TP+FN TN+FP TP+FP+TN+FN

11/14/19 106
• Sensitivity: The probability of testing
positive if the disease is truly present
Sensitivity = a / (a + c)
= TP X 100
TP+FN
• Specificity: The probability of screening
negative if the disease is truly absent
Specificity = d / (b + d)
= TN X100
TN + FP
• Accuracy= a+d/a+b+c+d
11/14/19 107
Predictive Value of a Screening Test

• Predictive value is the ability of a test to

predict the presence or absence of disease
from test results.

• Predictive Value of a Positive Test (PVPT)

or Positive Predictive Values shows the
probability that the person tested positive by
this specific test truly has the disease.
PVPT = TP X 100%
TP + FP

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 – Predictive Value of a Negative Test
(PVNT) or Negative Predictive Value
Shows the degree of confidence the
disease can be ruled out by using this
specific test.
PVNT = TN X 100%
TN+FN

11/14/19 109
• Predictive value of a test is determined by
Sensitivity, Specificity and the Prevalence of the
disease.
• The higher the prevalence, the more likely it is that a
positive test is predictive of the diseases i.e. PVPT will
be high.

• The more sensitive a test, the less likely it is that an

individual with a negative test will have the disease
and thus the greater the predictive value negative.

• The more specific the test, the less likely an individual

with a positive test will be to be free from the disease
and the greater the predictive value positive.

11/14/19 110
Reliability (Precision) of Screening Test

• A reliable screening test is one that gives

consistent results when the test is
performed more than once on the same
individual under the same conditions.

• Two major factors affect consistency of

results: the variation inherent in the
method and observer variation (observer
error).

11/14/19 111
1. The variability of a method- depends
on such factors as the stability of the
reagents used and fluctuation in the
substance being measured ( e.g in
relation to meals, diurnal variation).

2. Observer variation- can stem from

differences among observers (inter-
observer variation) and also from
variation in readings by the same
observer on separate occasions (intra-
observer variation).
11/14/19 112
• These variations can usually be reduced
by:

1. Careful standardization of procedures

2. An intensive training period for all
observers (or interviewers)
3. Periodic checks on their work
4.The use of two or more observers making
independent observations.

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 Evaluation of screening program

• Evaluation of a screening program

involves consideration of two issues:
– Whether the program is feasible, and
– Whether it is effective.

11/14/19 114
 Feasibility
The feasibility of a screening program is
determined by
• The acceptability of the program to the
potential screenees,
• Cost-effectiveness,
• The subsequent diagnosis and treatment
of individuals who test positive,
• The yield of cases.

11/14/19 115
 Feasibility

Persons with the disease

• Yield = detected by the test
X 100

Total screened

• Yield = TP X 100

TP + FN + TN + FP
11/14/19 116
 Effectiveness
 The evaluation of the effectiveness of a
screening program must be based on
measures that reflect the impact of a
program on the course of a disease.

 An effective screening program should

result in reduction of morbidity, mortality
and disability.

11/14/19 117
Exercise Diagnostic(Gold
standard Test)
Positive Negative Total

350 50 400
Screeni Positive
ng Test
150 450 600
Negativ
e

500 500 1000

Total
11/14/19 118
 Exercise .... Calculate and Interpret
from the above table
1. Sensitivity
2. Specificity
3. PPVT
4. NPVT and
5. Yield of the screening test

11/14/19 119
 THANK YOU!

11/14/19 120
 CHAPTER FOUR

EPIDEMIOLOGIC STUDIES

11/14/19 121
 Epidemiological design
strategies
o Epidemiology is primarily concerned with the
distribution and determinants of disease in
human populations

o The basic design strategies in epidemiologic

research are categorized into two according to
their focus of investigation

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 Epidemiologic approaches
1. DESCRIPTIVE
Used to describe health and disease in the
community
What? by….Who? When? Where?
What are the How many Over what Where do the
health problems people period of affected
of the community? are affected? time? people
live, work or
spend leisure
2. time?

ANALYTI
Used to identify etiology, prognosis and for
C Why? evaluation
program
How?
What are the causal By what
agents? mechanism
What factors affect
11/14/19 do they123
operate? 123
outcome?
11/14/19
124 124
 Descriptive studies

o Descriptive study is one of the basic types of

epidemiology describing the frequency and
distribution of diseases by time, place and
person

o Analytic studies focus in elucidating the

determinants of disease

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 What?

Cases
Person Time

Place

Who? Where?
When?
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126 126
 Category of descriptive
studies

 If population as study subjects

o Correlational /ecological studies

 If individual as study subjects

o Case report
o Case series
o Cross-sectional survey

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 Case report

o It is the study of health profile of a single

individual using a careful and detail
report by one or more clinicians
o Report is usually documented if there is
unusual medical occurrence, thus it may
be first clue for identification of a new
disease occurrences

o It is useful in constructing a natural

history of individual disease
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 Case series
o Individual case report can be expanded to a case
series, which describes characteristics of a
number of patients with the same diagnosis
o Similar to case report, it is usually made on cases
having new or unusual disease (giving interest to
clinicians)
o It is often used to detect the emergence of new
disease or an epidemic occurrences

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Example 1:-

o Five young, previously healthy homosexual men

were diagnosed as having Pneumocystis carinii
pneumonia at Los Angeles hospital during a six
month period from 1980 to 1981
o This form of pneumonia had been seen almost
exclusively among older men and women whose
immune systems were suppressed
o This unusual circumstance suggested that these
individuals
11/14/19 were actually suffering with130
a previously
130
Example 2:-
A 40-year old pre-menopausal woman developed
pulmonary embolism 5 weeks after beginning to
use an oral contraceptive preparation to treat
endometriosis.

What is unusual in this report? Pulmonary

embolism is common in older, postmenopausal
women. The investigator postulated that the drug
may have been responsible for this rare
occurrence
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 Advantages of case reports and
series
Used as bridge between clinical medicine and public
health

First clues in the identification of new disease or

adverse effect of exposure /drug

To identify outbreak occurrence or emergence of new

disease

Useful for constructing of the natural history of a

disease

Both
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case report and case series are able to
132 132
 Disadvantages of case reports and
series

Unable to test for statistical association between

exposure and outcome variables because of lack of
comparison group

Fundamental limitation of case report is inability to

avoid a roll of chance
Rates can not be calculated since the population
corresponding to the source of cases can not be well
defined
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 Ecological studies
o Uses data from entire population to compare disease
frequencies –
– between different groups during the same period
of time, or
– in the same population at different points in
time(TIME TREND)

o Unit of data source and unit of analysis is population

== > Population level exposure Vs population

level outcome

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 Strength
Can be done quickly and inexpensively, often using

available data.

May be best design to study health effects of

environmental exposures, eg
- Do heat waves increase death rate?
- Does soft drinks increase heart disease?
- Do economic recessions increase suicide rate?

Such questions only sensibly addressed at

population (or community) level

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 Limitation
1. Inability to link exposure with disease.
- Data on exposure and outcome are not linked at the
individual level;
- Correlation found with aggregate data may not apply
to individuals (this is referred as ecological fallacy)
2. Lack of ability to control for effects of potential
confounding factors
- A correlation found between the high per capita
color TV and mortality from CHD, again here it is
obvious that color TV owning is not a good reason for
increased mortality from CHD
3. It may mask a non-linear relationship between
exposure and disease while it is exist
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 Cross sectional study

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 Cross-sectional studies
It is also called prevalence study (survey study)

It is the major type of descriptive study designs

Survey is conducted in a population, to find

prevalence of a disease and risk factor at a point in
time

Exposure and disease status are assessed

simultaneously among individuals at a point in time

Point in time does not indicates the speed of data

collection i.e. Data collection process can take days,
weeks, months, years but the measurement takes
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Functions of cross-sectional studies
It helps administrators in assessing the health status
and health care needs of a population

The purpose is for effective health care planning,

priority setting, resource allocation and administration

Used to assess prevalence of acute and chronic

diseases, disabilities and utilization of health care
resources
- prevalence of common cold in kebele 8 of Gondar
town is 23%
- Prevalence of DM in kebele 8 of Gondar town is
10.45%
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 Example of cross-sectional study
undertaken in Ethiopia

• Census – house hold level

• Ethiopian demographic and health

survey(EDHS) – house hold level
• National immunization survey

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140 140
Advantages
Good design for hypothesis generation

Can estimate overall and specific disease prevalence

Can estimate exposure proportions in the population – proportion of

cigarette smokers in Gondar town, latrine utilization proportion

Can study multiple exposures or multiple outcomes or diseases-

malaria and diarrhea morbidity Vs proportion of latrine and ITN

utilization

Relatively easy, quick and inexpensive

Best suited to studying unchanged factors overtime (eye color, sex,

blood-type) – prevalence of malaria by blood type

Often good first step to employ analytical study designs

Highly generalizable
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 Disadvantage
Impractical for rare diseases and rare exposure – because we
need to take very large sample size

Not a useful type of study for establishing causal relationships –

chicken and egg dilemma

example- diarrhea Vs vitamin A defficiency = which comes first

Miss diseases still in latent period -

Recall of previous exposure may be difficult -

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 Analytical Study
Designs

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143 143
 Learning objectives

After the end of this session, students will

be able to:

o Describe purpose of analytical studies

o Differentiate observational and interventional
studies

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144 144
 Introduction to analytic
Application: studies
 To search for cause - effect relationship and
mechanism
o Why?

o How?

 It focuses on determinants of disease by testing

hypothesis regarding exposure and outcome of
interest
 To quantify the association between exposure and
11/14/19
145 145
outcome of interest
 Analytic studies...
Basic features:
 Key feature of analytic epidemiology is comparison
group
 Appropriate comparison group needed:
o Exposed versus non-exposed

o Case versus control

o Experimental versus non-experimental

 It is the use of comparison group that allows testing

of11/14/19
epidemiologic hypotheses
146 146
 Analytic studies...

Two types of analytic studies:

o Observational studies
o Interventional studies

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147 147
 Analytic studies...
Observational studies
o An investigator observes the natural course of an
event
o An investigator measures but does not intervene

Interventional studies
o An investigator assigns study subjects to exposed
and non-exposed and follows to measure for
disease occurrence
o An investigator manipulates the intervention
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 Observational analytic studies

Two basic observational analytic

studies:
o Cohort studies
o Case-control studies
o Cross-sectional studies?

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149 149
 Case-Control Studies

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150 150
 Definition of case-control
studies
 A case control study is one in which persons
with a condition (“cases”) and suitable
comparison subjects (“controls”) are
identified, and then the two groups are
compared with respect to prior exposure to
risk factors

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 Definition of case-control
studies…
o The investigator looks back in time to measure
exposure of the study subjects to the risk factors

o The exposure to the risk factors is then compared

among cases and controls

o To determine if the exposure to the risk factors could

account for the health condition of the cases
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11/14/19
153 153
 Steps of case-control studies
1. Define cases and controls
2. Identify group of cases
3. Identify group of controls
4. Asses both groups for previous history of exposure to risk
factors under study
5. Measure frequency of exposure to risk factors occurrence
in both groups
6. Compare frequency of exposure to risk factors between
cases and controls
7. Conclude that previous history of exposure to risk factors
contributed for the cases more than controls or not

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154 154
 What is case?
o It is the outcome of interest under study
o It can be:
– A disease
E.g. HIV status, malaria status
– A behavior
E.g. Alcohol drinking habit(yes/no),
cigarette smoking(yes/no)
- Event – traffic accident

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 What is control?
o It is the comparison group (reference)

o It should be free of the disease (outcome of interest

under study)

o It should be as similar as the cases in all aspects

except for the disease of interest under study

o Controls must have the same opportunity of getting

exposure to risk factors as cases and should be

subjected to the same inclusion and exclusion criteria
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 Selection of case and control
o Define ‘disease’ and how it will be measured

o Selecting the source population for cases (homogeneous cases)

o Sources of cases are commonly:

– Population based - All persons with the disease in a

population during a specific time of period

– Hospital based - All persons with the disease seen at a

particular facility (e.g. a hospital) in a specific time
period

– Incidents cases

–11/14/19
Chronic/prevalent cases
157 157
11/14/19
158 158
 Measuring exposure of
interest
o Exposure status could be ascertained by interview
(patient, relative), hospital records, laboratory
analysis etc…
o May include analysis of pre-diagnostic biological
specimens (nested case-control approach)

Comparison is made primarily by estimating the

relative risk as computed by the odds ratio.

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 Common bias in case-control
studies

o Information bias

- recall bias

- non-response bias
o Selection bias

- using different criteria to select cases and

controls
- the probability of selecting a real case and
control
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 Strengths of case-control studies
o To investigate rare disease (less than 10%)

o Suitable for the evaluation of diseases with long

latency period
o Quick with time and in-expensive

o Relatively efficient with small sample size comparing

with cohort studies
o No problem with attrition of study subjects

o Can examine multiple etiologic exposures for single

outcome
11/14/19
161 161
 Limitation of case-control
studies
o In-efficient for rare exposures

o In some situations, the temporal relationship

between exposure and disease may be
difficult to establish

o Prone to selection and information bias

o Selection of controls difficult some times

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 Cohort
Studies

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163 163
 Learning objectives

After the end of this session, students will

be expected to:

o Define cohort and cohort studies

o Describe purpose of cohort studies
o Define exposure and outcome of
interest
o List type of cohort studies
o State strengths and limitations of
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164 164
 What is cohort?
 Any designated group of persons who are followed or
traced over a period of time
Examples of cohort:
o Birth cohort
o Marriage cohort

o Immigration cohort

o Treatment cohort
o Exposure cohort

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165 165
 Definition of cohort studies
 A cohort study is an observational research design
which begins when a cohort initially free of disease
(outcome of interest) are classified according to a
given exposure and then followed (traced) over time
 The investigator compares whether the sub-sequent
development of a new cases of disease (other
outcome of interest) differs between the exposed and
non-exposed cohorts
 For example if a researcher want to investigate
weather drinking more than five cup of
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166 166
 Design of cohort studies
= == > If we want to know weather exposure to
drinking coffee during pregnancy will result in abnormal
birth Diseased
Exposed Give abnormal
Drink more baby
than five cup of
Coffee per day Not diseased
People
without Give normal baby
Population
at risk the Diseased
outcome Not Exposed Give abnormal
Pregnant baby
Not drink
mothers any coffee
Not diseased
Give normal baby
Time
Direction of enquiry
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167 167
Basic features of cohort studies
“Disease free” or “without outcome” population at entry

Selected by exposure status rather than outcome status

Exposure example – driving after drinking alcohol

- sleeping without using bed net

- feeding kids without washing our

hands

- not using glove during injection

Follow up is needed to determine the incidence of the outcome

Compares incidence rates among exposed against non-

exposed groups

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 Types of cohort studies
 Cohort studies can be classified depending
on the temporal relationship between the
initiation of the study and the occurrence of
the outcome of interest

– Prospective cohort studies

– Retrospective cohort studies

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169 169
11/14/19
170 170
 1. Prospective cohort studies
o Study participants are grouped on the basis of past

or current exposure status

o Both groups are followed into the future in order to
observe the outcome of interest
o At the beginning of the study the outcome has not
yet occurred
o Regarded as more reliable than the retrospective, if
the sample size is large and follow-up is complete

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171 171
 2. Retrospective cohort studies

o Both exposure and outcome status have occurred at

the beginning of the study

o Studies only prior outcomes and not future ones

o A historical cohort study depends upon the

availability of data or records that allow
reconstruction of the exposure of cohorts to a
suspected risk factor and follow-up of their mortality
or morbidity over time
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 Retrospective cohort studies…

o Suitable for studies of rare exposures, or where the

latent period between exposure and disease is long

o In other words, although the investigator was not

present when the exposure was first identified, s/he
reconstructs exposed and non-exposed populations
from records, and then proceeds as though s/he had
been present throughout the study

o What
11/14/19 is the difference b/n case control
173 and
173
 Which type of cohort studies
should be used?
 The decision to conduct a retrospective or
prospective study depends on:

o The research question – If very valid outcome is

required
o Nature of the outcome of interest – long/short latent
period
o Practical constraints such as time and money- if
yes/no…..
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174 174
o Availability of suitable study population and records
Data to be collected in cohort studies

o Data on the exposure of interest to the study hypotheses

–
o Exposure is any risk factor that can associated with the

occurrence of that disease

Example - chat chewing habit, alcohol drinking

habit
o Data on the outcome of interest to the study hypotheses

- Development of the disease of interest specific to the

exposure

11/14/19 Example- depression, 175

poor academic
175
Sources of exposure and outcome
information
o Pre-existing records- hospital records

o Information supplied by the study subjects-

interview

o Direct physical examination or screening tests

o Death certificate

o Laboratory sample analysis

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176 176
Follow up period of cohort studies
o The follow-up is the most critical and demanding part

of a cohort study.

o Lost to follow-up should be kept to an absolute

minimum (< 10-15%)

o Changes in the level of exposure to key risk factors,

after the initial survey and during the follow-up

period, are a potentially important source of random
bias
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 Ascertainment of outcome of
interest
• The aim of good case ascertainment is to ensure that the
process of finding cases, whether deaths, illness episodes,
or people with a characteristic, is as complete as possible

• Must have a firm outcome criteria and standard

diagnostic procedure which are equally applied for
exposed and non-exposed individuals

o Any outcome measurement should be done equally both

to the exposed and non-exposed groups

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Strength of cohort studies:
o Particularly efficient when exposure is rare

o Can examine multiple effects of a single exposure

o Minimize bias in outcome measurement if prospective

o Allows direct measurement of incidence (risk)

o Can elucidate temporal relationship between

exposure and outcome of interest (if prospective )

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179 179
 Limitation of cohort studies:
o Costly and time consuming if disease is rare and/or

long latency period (if prospective)

o Validity of the results can be seriously affected by

loss to follow up (if prospective)

o Relatively statistically inefficient unless disease is

common (need large sample size)

o If retrospective, requires availability of adequate

records
o Exposure status may change during the course of

study
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 Experimental
(Intervention studies)

By koku S. (BSc, MPH)

11/14/19 181
 Definition

An experiment is a set of
observations,
conducted under controlled
circumstances,
in which the scientist
manipulates the
conditions to ascertain what
effect such
manipulation has on the
11/14/19 182
 Key Features of
Experimental Design

1) Investigator manipulates
the condition under study

2) Always prospective

11/14/19 183
 Types of intervention studies

Generally considered either therapeutic or

preventive
1. Therapeutic (secondary prevention)
trials
conducted among patients with a particular
disease to determine the ability of an agent
or procedure to diminish symptoms,
prevent recurrence, or decrease risk of
death from that disease
 
11/14/19 184
Types of intervention studies cont..

2. Preventive (primary prevention)

trial
• involves the evaluation of whether
an agent or procedure reduces the
risk of developing disease among
those free from that condition at
enrollment
• Can be studied among either
individuals (Field trial) or entire
populations (Community trial).
11/14/19 185
 Intervention (Experimental)
Design
With
outcome
Received
Interventio Without
n outcome

Eligible B P
R
Individuals With
outcome
Not-
received
Without
Interventi outcome
on
R=randomization B=double blinding P=placebo
11/14/19 186
 Prevention Trial
With
outcome
Vaccinate
d Without
outcome

Health B P
R
Individuals With
Not- outcome

Vaccinate
Without
d outcome

R=randomization B=double blinding P=placebo

11/14/19 187
 Other ways of classifying intervention studies

A. Classification based on the

population studied
A.1 Clinical trial
• usually performed in clinical settings
• the subjects are patients
A.2 Field trial
• used in testing medicine for preventive
purpose
• subjects are healthy people e.g. vaccine
trial
A.3 Community trial
11/14/19 188
B. Classification based on design

B.1 Uncontrolled trial

• No control group
• Control will be past experience (history)
B.2 Non-randomized controlled
• There is control group
• Allocation to either group is not
randomized
B.3 Randomized controlled
• There is control group
• There is random allocation of subjects to
11/14/19 189
either group
C. Classification based on objective

C.1 Phase I
Trial on small number of subjects to
test a new drug with small dosage to
determine the toxic effect
C.2 Phase II
Trial on small group of people to
determine the therapeutic effect
C.3 Phase III
• Study on large population
• Usually randomized controlled trial
11/14/19 190
Problems of intervention studies

1. Ethical problem
• An independent group must be
established.

• Purpose:
– Safety of subjects
– Maintaining the quality of the study
– Maintain objectivity

11/14/19 191
2)Cost

• Intervention studies are expensive

3) Feasibility/ Practical Issues
•  Subject recruitment
– getting adequate individuals to enroll into the study is
not easy.
– Field trials particularly require greater number of
subjects since the risk of contracting a given disease for
the first time is small.

• Loss to follow-up
– Select population who are both interested and reliable.
– Arrange frequent contacts with individuals
– Use incentives, such as providing medical information

11/14/19 192
 Main steps in an RCT

1. Identify new
drug/intervention/prevention
2. Identify comparison – e.g standard
treatment versus placebo
3. Define eligible patient population/
exclusions (i.e the sampling frame)
4. Define the outcomes and how to
assess them
5. Write the protocol
11/14/19 193
 Main steps in an RCT cont…

6. Obtain research ethics committee

approval
7. Recruit & consent required sample
of patients
8. Randomise to treatment, then treat
9. Follow-up & compare/analyse
outcome data
10. Publish/disseminate findings
11/14/19 194
Selection of study
population

11/14/19 195
 Assessment of
compliance
• Self-report

• Pills count

• Biochemical tests

11/14/19 196
 Stopping Rules

• If the data indicate a clear and extreme

benefit,
• or if one treatment is clearly harmful, then
early termination of the trial must be
considered
• it would also seem unethical to stop a trial
prematurely based solely on the emerging
trends from a small number of patients.
• Such findings might well be only transient
and disappear or even reverse after data
have accumulated from a large sample 197
11/14/19
 Stopping Rules cont…

• the first requirement for even considering

modification or early termination of an
ongoing trial is the observation of a
sustained statistical association that is so
extreme, and there fore, so highly
significant, that it is virtually impossible to
arise by chance alone.
• The observed association must then be
considered in the context of the totality of
evidence
11/14/19 198
 Accumulation of Adequate
End Points
• Selection of a high-risk population

• Selection of adequate length of

follow-up period

11/14/19 199
 The Quality of “Gold
Standard"

• Randomization

• Blinding

• Use of Placebo

11/14/19 200
 Chapter Five

MEASURES OF ASSOCIATION
AND EVALUATION OF EVIDENCE

11/14/19 201
 Learning objectives
• After the end of this session, students will
be expected to:
– List common measures of association and
measures of public health impact
– Calculate and interpret
• relative risk
• odds ratio
• attributable and population attributable risk
and their percent

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 Definition of association
 An association is said to exist between two
variables when a change in one variable parallels
or coincides with a change in another ones
 An association is said to be causal when it can be
proved that a change in the exposure variable
produces a change in the outcome variable
 More appropriately, a causal relationship exists
when exposure enters into the causation of disease

11/14/19 203
o Statistical relationship between two or
more variables
o A causal association exists when the risk
factor causes change in the disease

11/14/19 204
 Measuring an association
 Quantifies the strength of the relationship
between an "exposure" and “outcome” of
interest
 Quantifies the difference in occurrence of
disease or death between two groups of
people who differ on “exposure status”

11/14/19 205
Two main options for comparison
 Calculate ratio of two measures of
disease frequency
o Relative comparison
o Strength of relationship between
exposure and outcome
 Calculate difference between two
measures of disease frequency
o Absolute comparison (attributable risk)
o Public health impact of exposure
(population attributable risk)

11/14/19 206
How strong is the association?
The strength of the association is
commonly measured by the relative risk,
odds ratio, attributable risk and population
attributable risk and their percents

11/14/19 207
 1. Relative Risk (RR) or Risk Ratio
 RR shows the magnitude of association
between exposure & disease
 Indicates the likelihood of developing the
disease in exposed group relative to those
who are not exposed
 RR can also be used to compare risks of
death, injury, and other possible outcomes
of the exposure.
11/14/19 208
 .
RR Incidence of a disease among exposed (a/
(a+b))
= Incidence of a disease among non-
exposed (c/(c+d)) Disease
. a .
Yes (+)
Exposu
RR = a+b re
Noa(-) b
a+
Yes (+)
.

b
.

.
.
c .
.
c d
c +d No (-) c+
d
– It is a direct measurement of a risk to
develop the outcome of interest
– It is usually used in cohort and
experimental studies
11/14/19 209
209
 Example
Table 1: data from a cohort study of oral
contraceptive (OC) use and bacteruria
among women aged 16-49 years
Bacteruria
Yes No Total
Current OC use
Yes 27 455 482
No 77 1831 1908
Total 104 2286 2390

11/14/19 210
 Calculate RR??????? Ans:- 1.4

 Interpretation: women who used oral

contraceptive had 1.4 times higher risk of
developing bacteruria when compared to
non-users.

11/14/19 211
 Interpretation

 The disease or health related out come is RR

times more likely to occur among the exposed to
the suspected risk factor than among those with
no such exposure.
 The larger the Value of RR, the stronger the
association between the disease in question and
exposure to the risk factor.
 Values if RR close to 1 indicates that the disease
and exposure to the risk factor are unrelated.

11/14/19 212
• Values of RR less than one 1 indicate a
negative association between the risk
factor and the disease.(i.e. protective )
• In general the strength of association can
be considered:
• High - if the RR is 3.0 or more

• Moderate – if the RR is from 1.5 to 2.9

• Weak – if the RR is from 1.2 to 1.4

11/14/19 213
 2. Odds Ratio (OR)
o Odds: The ratio of the probability of an
event's occurring to the probability of its
not occurring
o Odds Ratio: The ratio of two odds

11/14/19 214
 Cont’d…
 In case control studies, it is usually not possible
to calculate the rate of development of disease
in the exposed and non-exposed group.
 Hence, it is difficult to calculate the RR.

 The RR can be estimated, however, by

calculating the ratio of the odds of exposure
among the cases to that among the controls i.e.
the OR

11/14/19 215
• Odds Ratio: Odds of case being
exposed
Odds of control being exposed
OR = a/c = ad
b/d bc

• OR indicates the likelihood of

having been exposed among
cases relative to controls.

11/14/19 216
 Example
Table 3: Data from a case-control study of
current oral contraceptive (OC) use and MI in
pre-menopausal female nurses

Myocardial infarction
Yes No Total

Current OC use
Yes 23 304 327
No 133 2816 2949
Total 156 3120 3276

11/14/19 217
Calculate OR
OR = ad = (23) (2816) = 1.6
bc (304) (133)

Interpretation: the odds of

having MI is 1.6 times higher
among OCP users as compared
to that of the non OCP users.

11/14/19 218
 Interpretation of the Odds
Ratio…
• OR = 1 then exposure Not related to
disease
• OR >1 then exposure Positively
related to disease
• OR <1 then exposure Negatively related
to disease

11/14/19 219
 Interpretation
• The odds of having the disease in question
are OR times greater among those
exposed to the suspected risk factor than
among those with no such exposure.

11/14/19 220
 Exercise
• Suppose study was conducted in US in order to find
out weather mother’s use of hormone during
pregnancy influenced her son’s risk of testicular
cancer in later life. Investigator selected 500 peoples
with cancer and 1000 without cancer. The study
found 90 mothers in cases and 50 mothers in controls
had used hormones during pregnancy.
• What study design were used

• Prepare 22 Table

• Calculate appropriate measure of association

11/14/19 221
 Odds Ratio as estimator of
Relative Risk
OR is a valid estimator of RR if:

 Cases are incident and drawn from a known and

defined population

 Controls are drawn from the same defined

population and would have been in the case
group if they had the disease

 Controls are selected in an unbiased way

 the disease is rare

11/14/19 222
• What is the excess risk among
exposed individuals?

11/14/19 223
 Absolute Measures of Risk
o Absolute risk: a measure of association
indicating on an absolute scale how much
greater the frequency of diseases is in an
exposed group than in an unexposed group,
assuming the association between the
exposure and disease is causal
o Attributable risk is also known as risk
difference or excess risk among exposed
groups
11/14/19 224
 Attributable Risk (AR) / Risk
Difference (RD)
 Provides information about the absolute effect of
the exposure or the excess risk of disease in those
exposed compared with those non exposed.

 AR is the portion of the incidence of a disease in

the exposed that is due to the exposure.

 It is the incidence of a disease in the exposed that

would be eliminated if exposure were eliminated.

It takes into account the actual incidence rate of

the outcome.
11/14/19 225
It tells us how many cases of disease in
exposed people could have been prevented
by eliminating the exposure.

It is a measure of the impact of an

association on the exposed population.

AR = Incidence among exposed (Ie) -

Incidence among non-exposed (Io)

 For example in the study of OC use and

bacteruria:
AR=27/482 – 77/1908 = 0.01566 = 1566/10 5
11/14/19 226
 Cont’d…
 Thus, the excess occurrence of bacteruria
among OC users attributable to their OC
use is 1566 per 100,000.
 In other words, if we had prevented those
100,000 OC users from their use, we
would have prevented an estimated 1566
cases of bacteruria.
 AR is used to quantify the risk of disease in
the exposed group that can be considered
attributable to the exposure by removing
the risk of disease that would have
occurred anyway due to other causes (the
risk in the non-exposed).
11/14/19 227
 Cont’d…
 The interpretation of the AR is dependent
on the assumption that a cause-effect
relationship exists between exposure and
disease.
Discuss the interpretation when,
 AR = 0
 AR < 0
 AR >0

11/14/19 228
 Cont’d…
 AR=0 no association
 AR > 0 indicates positive association
 the number of cases of the disease among
the exposed that can be attributable to
the exposure itself, or
 alternatively, the number of cases of the
disease among the exposed that could be
eliminated if the exposure were eliminated
 Thus, the AR can be useful as a measure
of the public health impact of a
particular exposure.

11/14/19 229
• What proportion of cases is
attributed to the actual exposure
among exposed people?

11/14/19 230
Attributable Risk Percent (AR
%)
 Estimates the proportion of the disease among
the exposed that is attributable to the exposure,
or

 The proportion of the disease in the exposed

group that could be prevented by eliminating the
exposure.

 AR % = (Ie - Io)/ Ie X 100

= AR X100
Ie
11/14/19 231
 Example: Refer the previous table
and calculate AR%

• AR % = (Ie - Io)/Ie X 100

• AR % = 1566/100,000 X 100
27/482
= 27.96 %

Interpretation: If OC use causes bacteruria

(UTI), about 28 % of bacteruria among
women who use OC can be attributed to
their OC use and can be eliminated if they
did not use oral contraceptives.
11/14/19 232
 For most case-control studies, the AR
cannot be calculated
 It is, however, possible to calculate the AR%
using the following formula
 AR% = (OR – 1) x 100
OR
 Example: From the data on OC use and MI,
the OR of MI associated with current OC use
was 1.6, yielding AR% of 37.5%.
 If OC use causes MI, nearly 38% of MIs
among young women who used OCs could
be attributable to that exposure or could be
eliminated if they were to stop using Ocs.
11/14/19 233
• What is the excess risk among the
general population that is due to
exposure of interest?

11/14/19 234
 Population Attributable Risk
(PAR)
 Public health planners want to be able to
anticipate the effect of eliminating the
exposure on the population as a whole,
rather than just on the exposed part of the
population.

 Even if the RR is very high, eliminating a

very rare exposure would not be expected
to have much impact on the health of the
population as a whole.

 PAR takes into account not only the actual

incidence rate of the outcome but also the
prevalence rate of the exposure.
11/14/19 235
 Estimates the proportion of disease
occurring in the total population
attributable to the exposure.

 PAR is the portion of the incidence of a

disease in the population (exposed and
non exposed) that is due to exposure.

 It is the incidence of a disease in the

population that would be eliminated if
exposure were eliminated.

11/14/19 236
• PAR = AR X prevalence rate of the
exposure

Example: Research was conducted to

assess the association between cigarette
smoking and death from lung cancer.
• The following findings were obtained:
 AR = 89 per 100,000 per year

 Prevalence rate of cigarette smoking = 20

%
11/14/19 237
PAR = 89 per 100,000 per year X 20 %

= 17.8 per 100,000 per year

Interpretation:
In a population of 100,000 smokers, 18 deaths from
lung cancer per year could have been avoided by
preventing them from smoking (this refers to AR).
In a general population of 100,000 with a prevalence
rate of cigarette smoking of 20 %, about 18 deaths
from lung cancer per year would be prevented by
eliminating cigarette smoking (this refers to PAR).

11/14/19 238
 Both AR and PAR are used to estimate the
effect on disease incidence of eliminating a
given risk factor, but while AR estimates
reduction in disease incidence only in
those exposed, PAR estimates
reduction in disease incidence in the
population as a whole.
 The alternative formula for PAR is:
 PAR = Incidence rate in total population
minus incidence rate in non-exposed
population

PAR = IT - Io
11/14/19 239
• What proportion of cases is
attributed to the actual exposure
among the general population?

11/14/19 240
 Population Attributable Risk
Percent (PAR %)
 Expresses the proportion of disease in the study
population that is attributable to the exposure and
thus could be eliminated if the exposure were
eliminated.
 PAR% is the percent of the incidence of a disease in
the population (exposed and non exposed) that is
due to exposure.
 PAR % = PAR X100
incidence rate in total population
Example:
 PAR = 17.8 per 100,000 per year
 Mortality rate in non-smokers = 7 per 100000
 Mortality rate in the total population = 24.8 per 10 5
per year
 Calculate PAR %
11/14/19 241
• PAR % =17.8 per 105 per year X100
24.8 per 105 per year

=71.8%

• Interpretation: 72% of deaths from lung

cancer occurring in the general population
could be prevented by eliminating
cigarette smoking.

11/14/19 242
 Possible Outcomes In Studying The
Relationship Between Disease And
Exposure
1. No association between exposure and
disease
AR=0, RR=1 ,OR=1
2. Positive association between exposure
and disease (more exposure, more
disease)
AR>0, RR>1 ,OR>1
3. Negative association between exposure
and disease
(more exposure, less disease)
AR<0 (negative), RR
<1(fraction)OR<1

11/14/19 243
 Exercise
 There is some hint that coffee drinking causes
peptic ulcer. One epidemiologist wanted to
make sure whether this is true.

 He identified 600 people who drink coffee and

700 who do not drink coffee. Initially all the
study subjects were not suffering from peptic
ulcer. He followed them over 2 years period. In
this 2 years time 400 people who were drinking
coffee and 360 people who were not drinking
coffee developed peptic ulcer.

– What type of study design was used?

– Calculate and interpret the appropriate
measures of association

11/14/19 244
11/14/19 245
 EVALUATION OF EVIDENCE

Purpose of evaluation of evidence

• To determine if what is observed
is a reflection of the truth?
• Observed:

• incidence are they true?

• Prevalence
• Relative Risk Are there alternative
explanations?
• Odds Ratio…

11/14/19 246
 Brain storming

If exposure X is associated with outcome

Y…..then how do we decide if X is a cause of
Y or not?

11/14/19 247
 Association Vs Causation

 The existence of an association doesn’t

itself constitute a proof for causation
 An observed association could be a fact
or an artifact
 Hence, an association is a necessary but
not a sufficient condition for causation

11/14/19 248
 Accuracy
=
Validity + precision

Validity and reliability together makes up

accuracy
Validity - is the degree of closeness to the
truth
Reliability - is the degree of closeness
between repeated measurements of the same
thing
11/14/19 249
 Validity Vs Reliability

11/14/19 250
 Validity of epidemiological studies

o Validity is the extent to which a measured

value actually reflects truth
o Next step of evaluation of study results is
validation of the findings
o An observed association is validated for bias,
chance and confounding factors

11/14/19 251
 Types of validity

Internal validity:

=Is the degree to which a measured value is

true within the sample
External validity:

= Is the extent to which a measured value

apply beyond the sample(source population)
- This is related to generalizability

11/14/19 252
 Precision
 Precision measures the extent by which our study
result is similar (consistent) with other previous
studies published on the area by different scholars
in different population, area and methods.
 Precision is the extent to which random error alters
the measurement of effects
 Threats to validity of study:
- Random error (chance): is sampling error

- how we can avoid random error?

- Systematic error (bias): is error in the conduct of the
study
11/14/19 253
 Judgment of
Judgmentcausality
of causality is a process by which we
assure whether the observed association is
causal or not for that outcome
= Judgment of causality has two steps
1. Check whether the observed association
between exposure and disease is Valid (Rule
out chance, bias and confounding)
2. Check whether the observed association is
causal (Bradford hill criteria)

11/14/19 254
 The Bradford Hill Criteria
o It is the statement of epidemiological criteria
of a causal association formulated in 1965 by
Sir Austin Bradford Hill
1. Strength of association
2. Consistency of findings with other studies
3. Temporality
4. Biologic gradient
5. Biologic plausibility
6. Specificity of the association
7. Experimental evidence
11/14/19 255
 Bradford……….
1. Strength of the Association - The stronger the
association, the more likely that it is causal.
2. Consistency of the Relationship - The same
association should be demonstrable in studies with
different methods, conducted by different
investigators, and in different populations.
3. Specificity of the Association - The association is
more likely causal if a single exposure is linked to a
single disease.
Single exposure Single disease
This works more to most living organisms as causes.
11/14/19 256
 Bradford…….

4. Temporal Relationship - The exposure to the

factor must precede the onset of the disease.

5. Dose-response Relationship - The risk of disease

often increases with increasing exposure to a causal
agent
• 6. Study design- Study design;

It is most important to consider the design.

RCT > Cohort > Case control > Cross sectional

> Ecological >Case series/case report

11/14/19 257
 7. Biological plausibility
Hypothesis should be coherent with what is
known about the disease; both biologically
and using laboratory.

Knowledge about physiology, biology and

pathology should support the cause-effect
relationship

If a finding doesn’t go with what is known,

then it is losing biological plausibility

11/14/19 258
Which Hill’s criteria are essential?
There are no completely reliable criteria for
determining whether an association is causal or not
In judging the different aspects of causation; not all
criteria must be fulfilled to establish scientific
causation
The correct temporal relationship is more essential
risk factor/cause ---------------------
outcome/effect
=== Once this has been found, weight should be
given to:
– Strength of the association
– Biologic plausibility

– Consistency
11/14/19 259
– Dose-response relationship
 CHAPTER SIX

Public Health Surveillance

11/14/19 260
 Learning Objectives
After this session, students will be expected to:

 Define public health surveillance

 Discus the purpose of public health surveillance

 Describe types of surveillance

 List types of case definitions

 List criteria to select a disease for surveillance

system

 Discuss public health emergency and

management
11/14/19 261
 What is surveillance?

 Systematic, ongoing…
– Collection
– Analysis Health action
– Interpretation
investigation
– Dissemination
 …of health outcome control
data prevention

11/14/19 262
 Components of Surveillance and
Public Health Action

Public Health
Component of Action
Surveillance Priority setting
o Collection Planning,
o Analysis implementing and
o Interpretation
evaluating disease
o Dissemination
occurrence in that
community
11/14/19 263
 Purpose of Surveillance
I. To be able to identify diseases, injuries,
hazards and other health related factors as
early as possible. prediction and early
detection of outbreaks.
II. To provide scientific baseline data and
information for priority setting, planning,
implementing and evaluating disease
control program for both communicable
and non-communicable health problems.
III. To define the magnitude and distribution of
diseases by time, person and place
dimension
11/14/19 264
 Types of Surveillance

1. Active surveillance

2. Passive surveillance

3. Sentinel surveillance

11/14/19 265
 Active Surveillance

It is based on active case detection

Active case detection - is an active search for

cases by special surveys, house to house visits or

other methods outside of the routine health service

activities

It is not routine activities because it is expensive

However, it is more accurate and better

representative as community based data

E.g: Outbreak investigation and control

11/14/19 266
The advantages of active surveillance
 The collected data is complete and accurate
 Information collected is timely.
Disadvantages
 It requires good organization,
 It is expensive
 Requires skilled human power
 It is for short period of time(not a continuous
process)
 It is directed towards specific disease
conditions

11/14/19 267
Preconditions to institute an active surveillance
system.
 For periodic evaluation of an ongoing program
 For programs with limited time of operation such
as eradication program.
 In unusual situations such as .
– New disease discovery
– New mode of transmission
– When a high-risk season/year is recognized.
– When a disease is found to affect a new
subgroup of the population.
– When a previously eradicated disease reappears

11/14/19 268
 Passive Surveillance

 Based on passive case detection, and routine

recording and reporting activities

Passive case detection

 Cases detected in the course of the normal
operation of the health services, through the self
reporting of patients to health institutions
 The data is usually unreliable, incomplete,
inaccurate, unrepresentative and reported not
timely
11/14/19 269
Advantages of passive surveillance
Covers a wide range of problems
Does not require special arrangements
It is relatively cheap
Covers a wider area
Disadvantages
The information generated is to a large
extent unreliable, incomplete and inaccurate
 Most of the time, data from passive
surveillance is not available on time
 Most of the time, you may not get the kind
of information you desire
 It lacks representativeness as it is mainly
from health institutions
 There is no feed back system
11/14/19 270
 Sentinel Surveillance

Sentinel surveillance involves the collection of

case data from only part of the total
population (from a sample of providers) to
learn something about the larger population,
such as trends in disease
Under this strategy, health officials define
homogenous population and regions to be
sampled

11/14/19 271
 Sentinel Surveillance Con’t...
The advantages of sentinel surveillance data are
that they can be less expensive to obtain than those
gained through active surveillance of the total
population, and the data can be of higher quality
than those collected through passive systems.
In developing countries, sentinel surveillance
provides a practical alternative to population-based
surveillance

11/14/19 272
 Main Purposes of Sentinel Surveillance
 To detect changes

 To direct and focus control efforts

 To develop intervention strategies

 To promote further investigations

 To provide the basis for evaluating

preventive strategies

11/14/19 273
Advantages of sentinel surveillance
 Relatively inexpensive
 Provides a practical alternative to population-
based surveillance
 Can make productive use of data collected for
other purposes
Disadvantages:
 The selected population may not be
representative of the whole population
 Use of secondary data may lead to data of lesser
quality and timeliness

11/14/19 274
 Process of Public Health Surveillance

 Data collection and recording

 Reporting and notification
 Data compilation
 Data analysis
 Data interpretation
 Data dissemination
 Link to public health action

11/14/19 275
Criteria for selecting and prioritizing health
conditions for surveillance system
includes:
 Public health importance of the problem:-
• incidence, prevalence,
• severity, sequela, disabilities,
• mortality caused by the problem,
• socioeconomic impact,
• communicability,
• potential for an outbreak,
• public perception and concern, and
• international requirements.
11/14/19 276
 If disease has epidemic potential

 If required internationally

 Have available effective control and prevention

programs

 Can easily be identified using simple case

definitions

11/14/19 277
 Basic Principles of Surveillance

• Public health surveillance main function is

to serve as an “early warning system”
providing timely information needed for
action (rapid reporting, confirmation,
decision making and response)

11/14/19 278
 Elements of Surveillance
• Case definition

• Population under surveillance

• Cycle of surveillance
• Confidentiality

• Incentives

11/14/19 279
 A) Case Definition

Case definition: is a set of criteria for deciding

whether an individual should be classified as having
the health condition of interest or if the case can be
considered for reporting and investigation

Case definition for cholera

- Acute watery diarrhea: three or more abnormal

loose or fluid stool within 24 hours with or without
dehydration

11/14/19 280
 Standard case definition
 If the use of case definition is agreed by
everyone in the country or across boundaries
or continents it is standard case definition
o Surveillance using less specific criteria is
sometimes referred as syndromic surveillance

E.g: Polio (AFP)

11/14/19 281
 Types of Case Definition

Confirmed case
o A case with laboratory verification
Probable case
o A case with typical clinical features of the
disease without laboratory confirmation
Suspected case/possible
o A case presented with fewer of the typical
clinical features of the disease without
laboratory confirmation

11/14/19 282
 Advantages of Case Definition
1) Facilitate early detection and prompt
management even if diagnosis is not
confirmed by laboratory

2) Laboratory test is expensive, difficult to obtain

3) Observation of trends

4) Comparison from one area to another

11/14/19 283
 B) Population Under Surveillance

Target population can be:

• Individuals at specific institutions
• Residents of a community
• Residents of a nation, etc.

11/14/19 284
 C) Cycle of Surveillance

11/14/19 285
 D) Confidentiality
• Personal identifying information is necessary to:

- identify duplicate reports

- obtain follow-up information when necessary

- provide services to individuals

- use surveillance as the basis for detailed

investigations
• Protecting the physical security and confidentiality of
surveillance records is both an ethical responsibility
and a requirement for maintaining the trust of
participants
11/14/19 286
 E) Incentives to Participants
Providing information back to those who contribute to
the system

• This feed back may be in the form of reports and

seminars

• Other incentives may be more immediate, such as

payment for case reports

• Individuals who participate may be paid for their time

and willingness to provide blood or other specimens

11/14/19 287
 Public Health Emergency Management (PHEM)
 Public health emergencies are events or disasters that
threaten the health of communities at large

Some examples are disease outbreaks (emerging and

reemerging) and pandemics, natural disasters such as

earthquakes, floods, droughts, volcanoes; biological
terrorist attacks such as an anthrax release

PHEM is the process of anticipating, preventing,

preparing for, detecting, responding to, controlling and
recovering from consequences of public health threats in
order that health and economic impacts are minimized
11/14/19 288
E m e rg e nc y M a na g em e nt Ph ases
 Mitigation
 Pre-event planning and actions which are
intended to lessen the impact of a potential
disaster

E.g: Risk identification / assessment and

reduction

11/14/19 290
 Preparedness
 Actions taken before an emergency to prepare for
response

 Develop emergency management plan

 Develop communication plan

The identification of a public health threat by closely

and frequently identified monitoring indicators and

predicting the risk it poses on the health of the
public and the health system (early warning phase)

11/14/19 291
 Response
 Activities to address immediate and short-
term effects of a disaster
– Implement emergency management plan
o Save lives

o Meet basic human needs

11/14/19 292
 Recovery
 Restore essential functions and normal operation

– Assess damage / impact of disaster

– Address psychological needs of patients and

community

– Produce after action debriefing and report

11/14/19 293
Limitations of Surveillance System In Ethiopia

o Under reporting

o Lack of representativeness

o Lack of timeliness

o Inconsistency of case definitions

o Shortage of qualified staff

o Lack of motivation

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 List of Priority Reportable Diseases In Ethiopia

Diseases targeted for

eradication
Epidemic-prone
diseases 12. Acute flaccid paralysis (Polio)
1. Cholera 13. Dracunculiasis (Guinea worm)
2. Diarrhea with blood14. Leprosy
(Shigellosis) 15. Neonatal tetanus
3. Measles Diseases of public health
4. Meningitis importance
5. Plague 16. Pneumonia in children
6. Viral hemorrhagic 17. Diarrhea in children
fever 18. New AIDS cases
7. Yellow fever 19. Onchocerciasis
8. Typhoid fever 20. Sexually transmitted diseases
9. Relapsing fever 21. Tuberculosis
10. Epidemic typhus 22. Rabies
11. Malaria 23. Emerging unknown liver
disease?
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 Chapter seven

Outbreak Investigation & Control

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 Learning objectives
After the end of this session, students will be able to:

o State different level of disease occurrences

o List the rationale to investigate outbreak occurrence

o Describe types of outbreak occurrence

o Discuss steps in the investigation of an outbreak

occurrence
o Discuss the outbreak controlling strategies

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 Level of disease
occurrences
• Diseases occur in a community at different levels at a
particular point in time.
Occurrence at expected levels
• Endemic: Presence of a disease at more or less stable level.
--==Malaria is endemic in the lowland areas of Ethiopia.
• Hyper endemic: Persistently high level of disease occurrence.

• Sporadic: Occasional or irregular occurrence of a disease

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 Excess of what is expected
• Epidemic: The occurrence of health related
condition/disease in excess of the usual frequency
• Outbreak: Epidemics of shorter duration covering
a more limited area.
• Cluster: is an aggregation of cases in a given area
over a particular period without regard to whether
the number of cases is more than expected.
• Pandemic: An epidemic involving several
countries or continents affecting a large number
of people.
example : HIV/AIDS is a pandemic.

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 What is outbreak
occurrence?

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 What does outbreak
investigation & control?
 It is the process of identifying:
o The cause of the epidemic

o The source of the epidemic

o The mode of transmission and

o Taking preventive and control

measures
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 Source of an outbreak information/how
to recognize epidemic

 Routine surveillance (Recently WHO recommends

global alert and response system or GAR)

 Health professionals (reporting from healthcare

settings, laboratories, and pharmacies)

 Affected community members

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 What are the objectives for
outbreak investigation? Or reason
to investigate
1)To initiate control & prevention measures/to
evaluate existing preventive strategies i.e.
vaccine
 The most important public health reasons for
investigating an outbreak are to help guide disease
prevention and control strategies.
 These disease control efforts depend on several factors,
Including
 knowledge of the agent,
 The natural course of the outbreak,
 The usual transmission mechanism of the disease,
and
 Available control measures

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 2) Research and training
opportunity
o Each outbreak should be viewed as an experiment
waiting to be analyzed
o It presents a unique opportunity to study the
natural history of the disease
o It could be a good opportunity to gain additional
knowledge on
– The impact of prevention and control measures

– The usefulness of new epidemiology and

laboratory techniques

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 3) Public, political and legal
obligations
o Politicians and leaders are usually concerned with
control of the epidemic or address public concern
about the outbreak.
o Politicians and leaders may sometimes override
scientific concerns
o The public are more concerned in cluster of
disease and potentials of getting medication
o It is the right of the community to get
treatment/service and it is government and our
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duty to protect the community
 4) Program
considerations
o Occurrence of an outbreak notifies the
presence of a program weakness
o This could help program directors to change
or strengthen the program’s effort in the
future to prevent potential episodes of
outbreak occurrence

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Goals of investigation
Identify the etiologic agent

Identify the reservoir(s)

Identify the mode of transmission

Apply control and preventive measures

Eliminate the reservoir(s) and

transmission
Prevent future outbreaks
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 Types of Epidemics
 There are three principal types of epidemic

1.Common source – based on source of

exposure

2.Propagative - touches mode of transmission

3.Mixed epidemic – share characteristics of

both type

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 1. Common source
It occurs as a epidemic
result of the exposure of a group of
population to a common source (etiological agent)
o It can result from a single source/ exposure of
the population to the agent
E.g: contaminated water supply, or food in a
certain restaurant
Three types
1. Point common source
2. Continuous common source
3. Intermittent common source
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 A) Point common source
epidemic
o Single/ones/limited time exposure to the source
o All exposed hosts will develop disease within one
incubation period
o The epidemic usually decline after a few
generations, either because the number of
susceptible hosts fall below some critical level, or
because intervention measures become effective
o A rapid rise and gradual fall of an epidemic curve
suggests a point source epidemic
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 E.g. Food borne outbreak in a
wedding feast

Commonly due to
infectious diseases or
May be from
environmental
pollution Features of epidemic curve:
1-Rises and falls rapidly, no
secondary waves.
2-Tends to be explosive, with
clustering of cases within narrow
interval of time. 3-All cases
develop within one incubation
period.

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 B) Continuous common source
epidemic
 If exposure to a common source continues over
time for days, weeks
 The epidemic curve has a plateau (multimodal
epi curve)/ long peak
 Range of exposures and range of incubation
periods is different
1-The exposure from the same
source may be prolonged-
continuous, repeated or
intermittent
2-No explosive rise in number
of cases.
3-Cases occur over more than
one incubation period.

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 C) Intermittent common source
epidemic

 Results in an irregular pattern of the

epidemic curve that reflects the
intermittent nature of the exposure

E.g. waterborne outbreak

 Often the graph is atypical

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 2. Propagative /progressive
o
epidemic
It occurs as a result of transmission of an
infectious agent from one person to another
directly or indirectly
o There is a successive generations of cases

o The epidemic curve in a progressive epidemic is

usually presence of successive several peaks, a
prolonged duration, and usually a sharp fall
o Can show geographic spread of the case

Example; Malaria outbreak and different

vector born disease
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 Propagated Epidemics

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 Typical Propagated Epidemic Curve

Of infectious origin, with person to person transmission (hepatitis A,E and polio epidemics).
Gradual rise and tails off over longer period of time. Transmission continues till depletion of
susceptible or susceptible individuals are no longer exposed to source of infection. Communicability
(speed of spread) depends on herd immunity among exposed and opportunities for contact with
infective dose and secondary attack rate.

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 3. Mixed Epidemic
o It shows the features of both types of epidemics

o It begins with a common source of infectious agent

with subsequent propagated spread because of person
– to- person transmission of the etiologic agent

E.g. Majority of food borne outbreaks

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 Steps of outbreak investigation and control
1.Prepare to field work
2.Establish the existence of outbreak
3.Verifying the diagnosis
4.Case definition and case finding
5.Perform descriptive epidemiology
6.Formulate hypotheses
7.Testing hypotheses
8.Refine hypothesis and additional studies
9.Implementing prevention and control activities
10.Communicate findings
 In practice, however, several steps may be done
at the same time, or
 The circumstances of the outbreak may dictate
that a different order be followed
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 Step 1: Prepare for field
work
 Before leaving for the field, an investigator
must be well prepared to under take the
investigation:
o Investigation (Knowledge in epidemiology
and the disease of concern is important)
o Administrative (Logistics, administrative
procedures, travel arrangements)
o Consultation (Health workers should know
their role, and should participate in the
planning phase)

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 Step 2: Establish the existence of
outbreak
o An outbreak is the occurrence of more cases of
disease than expected level
o The investigator has to compare previous case
load with the current to assure the existence
of the outbreak
o But be careful, excess cases may not always
indicate an outbreak occurrence rather it may
be because:
 Change in population size
 Change in case definition
 Change in reporting procedure

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Step 3: Verifying the diagnosis
o The initial report may be spurious and arise from
misinterpretation of the clinical features
o Review clinical and laboratory findings to establish
diagnosis
o Goals in verifying the diagnosis includes:

 To ensure that the problem has been properly

diagnosed
 To rule out laboratory error as a basis for the
increase in diagnosed cases
 To ensure the diagnosed disease is possibly epidemic

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 Step 4: A. Case definition and
case finding
• Define cases ( Establish case definition):

• Case definition should be broad enough to include most,

if not all, of the actual cases. (sensitive not specific)
• Case definition must not include an exposure or risk
factor you want to test
• Case definition must be equally applied and without bias
to all persons under the investigation

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Step 4: Case definition and case
finding Cont…d

• Usually includes four components:

 Clinical information about the disease, 

 Characteristics about the people who are affected, 

 Information about the location or place, and 

 A specification of time during which the outbreak

occurred

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 Define cases ( Establish case definition):
1)Possible (suspected)
 Having fewer sign and symptoms
2) Probable
 Having typical sign and symptoms
3) Definite (confirmed)
 Laboratory confirmed
Example : A patient hospitalized in the ICU from
24th November 2017, with new or worsening of
cough, Fever >38, with suggestive X-ray changes
and cultures identify a respiratory microorganism.

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 B. Identify cases (line listing)

o Identifying information
e.g. Hospital admission number, unit, name, address,
phone.
o Demographics
e.g. Age, sex, date of admission, date of surgery.
o Risk factor information
e.g. Type of surgery, co morbidity, catheters, implants
o Clinical data
• e.g. Onset of symptoms and signs, frequency and
duration of clinical features, treatments, devices, etc

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 Step 5: Performing Descriptive
Epidemiology
o Once data is collected, it should be analyzed by time, place and
person

o The tools to be used when characterizing the epidemic are epidemic

curve, spot map and attack rate

o The characterization often provides clues about etiology, source and

modes of transmission that can be turned into testable
epidemiologic hypothesis

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1. Analysis of epidemic by time

 We use epidemic curve to analyze by time

taking
- The X- axis; indicating time of onset
-The y-axis; indicating the number of
cases appearing
 Epidemic curve can tell as
- nature of epidemic
- hint to etiologies – etiologic agent
- hint about source of exposure

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 Epidemic curve

Point source

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 2. Analysis of epidemic
byisplace
– A spot map a simple and useful
technique for illustrating where cases
live, work or may have been exposed
– Area map if large area is affected

– It is important to indicate source of

outbreak

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 John Snow’s spot map of the distribution of cholera
cases Golden London square August-September 1848

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 3. Analysis of epidemic
by person
o Characterizing an outbreak occurrence by person is how we
determine what populations are at risk for the disease
o Host characteristics: age, race, sex, or medical status and
exposures-occupation, leisure activities, use of medications,
tobacco and drug use etc…
o These influence susceptibility to disease and opportunities
for exposure to risk factors
o We use attack rates to identify high risk groups

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 Attack rate

• Number of patients affected divided by the total number of population

at risk
• Number of infections divided by the total number of population at risk

• Number of adverse outcomes divided by the total number of

population at risk
• Computing relative risk (ratio of incidence among exposed to among
non exposed)

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 Step 6: Formulating Hypothesis
 Depending on the outbreak, the hypothesis may address
o The exposures that caused the disease
o The agent and its reservoir
o Risk factors that caused disease
o The mode of transmission
 Using :
1. Subject-matter knowledge
2. Descriptive epidemiology
3. Talking with patients
4. Talking with local officials
 The hypotheses should be testable

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 Step 7: Testing the hypothesis
 Evaluate the credibility of your hypotheses

 Use analytic epidemiology and appropriate

measure of association(case control, odds ratio for
cohort design, relative risk)

 Significance of statistics should be done, (Chi-

square is the appropriate test, and P-value is

estimated at 5 %) to explore the role of chance

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 Factors that should also be
considered when evaluating
possible causality:
• Testing statistical significance
• Consistency with other studies

• Temporality. Exposure to the factor precedes onset of disease.

• Biologic plausibility. Does the association make sense
biologically?

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 Step 8: Refining hypotheses and
o
additional studies
When analytic epidemiological studies do not confirm

hypotheses, we need to reconsider hypotheses and

look for new vehicles or modes of transmission.

o Sometimes you will need to refine your hypotheses

to obtain more specific exposure histories or a more

specific control group.

o Laboratory and environmental studies

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 Step 9: Implementing control and
prevention
In outbreak investigation, the
primary goal is to control and
prevent the outbreak.
Implementing control measure
should be done as soon as possible
It should go in parallel to
investigating the outbreak

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 Source/ Mode of Transmission

Causative
Agent

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 Control measures (do early)
1. Measures Directed Against the Reservoir:
– Reduce contact rate
– Reduce infectious sources- destruction of infected animal
/isolation
– Reduce infectiousness- early treatment

2. Measures that interrupt the transmission of organisms

– Purification of water
– Pasteurization of milk
– Inspection procedures designed to ensure safe food supply.
– Improve housing conditions

3. Measures that reduce host susceptibility and Increase herd immunity

- Immunization

- Chemoprophylaxis - Use of antibiotics for known contacts of cases

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 DAY

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 DAY

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 Step 10: Communicating
findings of investigation

 The final responsibility of the investigative team is to prepare a written report to document the
investigations, findings and the recommendations

The written report should follow the scientific reporting format which includes:
o introduction

o methods
o results
o discussion
o conclusion, and
o Recommendations

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 Summary of the investigation and control
of an epidemic considering procedure

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 Post-Epidemic
Surveillance
o The efficacy of control measures should be assessed day to
day during the outbreak, a final assessment being made
after it has ended
o This will provide a logical basis for post-epidemic
surveillance, and preventive measures aimed at avoiding
similar outbreaks in the future
o Develop long term early warning system

o Monitor environmental risk factors

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 The End!

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