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Pediatr Infect Dis J. Author manuscript; available in PMC 2019 February 01.
Published in final edited form as:
Pediatr Infect Dis J. 2018 February ; 37(2): 138–143. doi:10.1097/INF.0000000000001750.
Low Varicella Zoster Virus Seroprevalence Among Young
Children in the Democratic Republic of the Congo
Reena H. Doshi, PhD, MPH1,*, Vivian H. Alfonso, PhD, MPH1,*, Patrick Mukadi, MD2, Nicole
A. Hoff, PhD, MPH1, Sue Gerber, MPH3, Ado Bwaka, MD4, Stephen G. Higgins, MS5, Brian
Cowell, PhD, MS6, Guillaume Ngoie Mwamba, MD7, Emile Okitolonda, MD, PhD8, JeanJacques Muyembe-Tamfum, MD, PhD9, and Anne W. Rimoin, PhD, MPH1
Author Manuscript
1Department
of Epidemiology, University of California, Los Angeles, Los Angeles, CA
2Department
of Microbiology, Kinshasa School of Medicine, Kinshasa, DRC
3Bill
and Melinda Gates Foundation, Seattle, WA
4McKing
Consulting, Expanded Programme on Immunization, Kinshasa, DRC
5OpGen,
Inc. Gaithersburg, MD
6Department
of Social and Behavioral Health, School of Rural Public Health, Texas A & M
University, College Station, TX
7Programme
8Kinshasa
Author Manuscript
9National
on Immunization, Kinshasa, DRC
School of Public Health, Kinshasa, DRC
Institute for Biomedical (INRB), Kinshasa, DRC
Abstract
Background—Varicella zoster virus (VZV) causes both varicella (chickenpox) and herpes zoster
(shingles) and is associated with significant global morbidity. Most epidemiological data on VZV
come from high-come countries and to date there are limited data on the burden of VZV in Africa.
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Methods—We assessed the seroprevalence of VZV antibodies among children in the Democratic
Republic of Congo in collaboration with the 2013–2014 Demographic and Health Survey (DHS).
Dried blood spot samples collected from children 6–59 months of age were run on Dynex™
Technologies Multiplier FLEX® chemiluminescent immunoassay platform to assess serologic
response. Multivariate logistic regression was then used to determine risk factors for VZV
seropositivity.
Results—Serologic and survey data were matched for 7,195 children 6 to 59 months of age,
among whom 8% were positive and 2% indeterminate for VZV antibodies in weighted analyses.
In multivariate analyses, the odds of seropositivity increased with increasing age, increasing
socioeconomic status, mother’s education level, rural residence, and province (South Kivu, North
Kivu, Bandundu, Bas Congo had the highest odds of a positive test result compared to Kinshasa).
Corresponding author information: Anne Rimoin, Ph.D., M.P.H., Department of Epidemiology, UCLA Fielding School of Public
Health, 650 S Charles E Young Drive, Los Angeles, CA, 90095, USA, arimoin@ucla.edu, Phone: (310) 825-2096.
*These co-first authors contributed equally to this work.
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Conclusion—Our data suggest that VZV is circulating in DRC and seropositivity is low among
children 6 to 59 months. The with prevalence of seropositivity increased with age and varied by
other sociodemographic factors, such as geographic location. This study provides the first
nationally representative estimates of varicella zoster virus infection among children in the DRC.
Keywords
varicella zoster virus; varicella; herpes zoster; immunization; vaccine-preventable diseases;
Democratic Republic of the Congo
Introduction
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Varicella zoster virus (VZV) is the causative agent of both varicella (chickenpox) and herpes
zoster (shingles), both of which occur worldwide (1). Primary varicella infection is
characterized by a vesicular rash and fever, usually presenting as a mild, self-limiting illness
in children (2). The most common complication in young children is secondary bacterial
infection (3, 4); however, complications are more likely to occur in infants and adults,
including pregnant women and immune-compromised individuals (5, 6). VZV is highly
contagious with secondary attack rates in susceptible populations ranging from 61 to 100
percent (7–11) and is typically transmitted person-to-person by direct contact with lesions or
inhalation of aerosolized droplets (1, 12).
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Varicella is reported worldwide, however, population-based data are generally available from
high-income countries (13–16). Studies conducted before widespread usage of the varicella
vaccine in temperate regions suggest more than 90% of children are immune to VZV by
adolescence due to natural infection (17–20). Varicella remains a public health concern
among countries that do not have widespread vaccination coverage: contrary to findings in
developed countries with temperate climates, limited data from the developing world
including Thailand (21, 22) and India (16) suggest that varicella infection is less common in
young children as infection tends to occur at a later age (13). This shift in the age of
infection in tropical regions is also associated with markedly increased severity and
complications such as skin infections, encephalitis, pneumonia, meningitis and death (16,
22, 23). On the other hand, Guinea Bissau, a developing, tropical country, has a median age
of infection of 4 years (24) and by 10 years of age, 90% of the population is infected (25).
The reasons for the differing age epidemiology in various climates remain poorly understood
(24); possible reasons include properties of VZV, climate, population density and the risk of
exposure (22, 26).
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In the Democratic Republic of the Congo (DRC), varicella is not a reportable disease, thus
the burden of infection remains unknown (27). Therefore, in collaboration with the 2013–
2014 DRC Demographic and Health Survey (DHS), we set out to obtain nationally
representative, age-specific estimates of varicella infection among children 6 to 59 months
of age.
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Methods
Study Population and Design
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The DRC is the largest country in sub-Saharan Africa and the fourth most populous nation
on the continent with an estimated 77.8 million inhabitants, 11.9 million of which are under
the age of five (28). At the time of this study, the DRC was administratively divided into 11
provinces. The country lies directly on the equator with one-third of the country to the north
and two-thirds to the south. In the equatorial climate of Equateur, Orientale and Maniema
provinces, the climate is generally tropical and moist, whereas in Kinshasa, Bas Congo,
Bandundu, Kasai Oriental, and Kasai Occidental provinces, the climate is considered
tropical savanna. In the southern highlands of Katanga, the climate is humid and subtropical.
The eastern highlands of North and South Kivu generally have a cooler, alpine climate.
North and South Kivu have also experienced the largest security difficulties and attacks by
rebel groups, but have received more humanitarian interventions (29).
From November 2013 to February 2014, the second DHS was conducted in the DRC. Using
a 2-stage stratified cluster design, the survey generates nationally representative data on
population health and social indices. Details on the sampling design and data collection
procedures are described elsewhere (30). Interviews were conducted among a nationally
representative sample of 18,141 households. After parental consent, dried blood spots (DBS)
were collected from children aged 6 to 59 months in 50% of the households selected.
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All survey data were double entered from paper questionnaires to an electronic format, using
the Census and Survey Processing System (U.S. Census Bureau, ICF Macro), and verified
by comparison. Ethical approval was obtained at UCLA Fielding School of Public Health,
the Kinshasa School of Public Health and the Centers for Disease Control and Prevention.
Laboratory Analysis
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DBS samples were extracted using a modified extraction protocol (31) and processed at the
UCLA-DRC laboratory housed at National Laboratory for Biomedical Research (INRB) in
Kinshasa, DRC. A 0.25" DBS punch was extracted, shaken at room temperature in 1ml
phosphate buffered saline, 0.05% Polysorbate 20, and 5% dried milk, which represents a
1:143 fold dilution assuming 7µl of serum per punch. The Dynex™ Technologies Multiplier
FLEX® chemiluminescent immunoassay platform with a research use-only M² multiplex kit
for measles, mumps, rubella, varicella-zoster virus, and tetanus (MMRVT) was used to test
samples for IgG antibody response. Polystyrene beads coated separately with antigen to
measles, mumps, rubella, varicella-zoster, and tetanus were immobilized within 54-well M²
assay strips with 10 beads per well and processed using a modified Dynex DS2® automated
ELISA system for IgG antibody detection.
Cut-offs were calculated via receiver operating characteristic (ROC) analysis to maximize
agreement between 4 commercially available FDA-cleared ELISA kits (32) and
categorization of serologic results were as follows: <0.18 IU/mL as negative; 0.18 to < 0.30
IU/mL as indeterminate; ≥0.30 IU/mL as positive. For analyses, the positive/negative cut-off
for VZV IgG antibody detection was set at 0.30 IU/mL.
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Statistical analysis
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Chi-square analyses were performed on the weighted sample (using survey weights) to
examine sociodemographic differences by serologic test results (positive and negative for
varicella-zoster antibody). Univariate logistic regression models were used to identify
independent predictors of seropositivity among variables included in Table 1. Multivariable
regression models were initially run with all variables; using backward selection only
significant predictors (alpha ≤.05) were retained. Children for whom test results were
indeterminate (n=125) were categorized as serologically positive and in sensitivity analyses
these subjects were removed to assess the impact of their inclusion on findings. All
statistical analyses were performed using SAS 9.4 (SAS Institute, Cary, NC) and maps were
created using ArcGIS software version 9.3 (ESRI, Redlands, CA).
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Results
Among the 7,513 eligible 6–59 month olds, approximately 96% (n=7,195) were linked to
seroprevalence data and included in analyses. Among these children, 572 (8%) were
positive, 6,498 (90%) were negative, and 125 (2%) were indeterminate for varicella-zoster
antibody (Figure 1). Categorizing children with indeterminate results as positive and
accounting for population sampling methods through the application of DHS sampling
weights, a total of 815 children (11%) were positive and 6,435 (89%) negative (Table 1).
Overall, children born to higher socioeconomic status (SES), and older mothers living in
urban environments were more likely to test positive for varicella zoster virus IgG compared
to their younger, lower SES, rural counterparts.
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Furthermore, assessment of the assay response reveals an increase in seropositivity with
increasing age: from 2% of 6 month olds to 19% of 59 month olds (range: 2%–23%) (Figure
2). Among all children, those living in North and South Kivu had the highest prevalence of
seropositive children (17% and 24%, respectively), while Equateur and Maniema had the
lowest (4% for both) (Figure 3). This pattern in serologic response according to geographic
region persisted in those ≥1 year of age. However, among those 6–11 months of age, South
Kivu and Kinshasa had the highest prevalence of positive results (18% and 11%,
respectively) while North Kivu and Katanga had the lowest (<1% and 1%, respectively). In
multivariate analyses, the odds of seropositivity increased with increasing age, number of
children residing in the household, wealth index, and province (Table 2). Sensitivity analyses
excluding indeterminate serologic results did not impact findings (data not shown).
Discussion
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These findings suggest that VZV is circulating in DRC with an overall seroprevalence of 8%
among children 6–59 months of age. We also observed a trend of increasing seropositivity
with older age and varying serologic response by geographic region (North and South Kivu
have the highest prevalence of VZV-positive children). These findings provide the first
nationally representative estimates of varicella zoster virus infection among young children
in the DRC.
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Our findings of low infectivity among young children and increasing seroprevalence with
older age are consistent with other studies (16, 33). Between 2001–2004, the monkeypox
active disease surveillance system estimated the average age of VZV infection to be 20.6
years among the 61 positive participants(33). While the study was not nationally
representative, it does suggest VZV exposure occurs at older ages. Our estimate of VZV
seroprevalence among children less than 5 years of age in the DRC is lower than other
studies in tropical regions (16). However, estimates of seropositivity among young children
differ significantly by country: estimates range from 16% of children 1–4 years of age in
India (34); 24% of children 1–4 years of age in Thailand (21); 26% of children 1–5 years of
age in Malaysia (35); 30% of children under 5 years of age in the Philippines (23); and 35%
of children 1–6 years of age in Singapore (36). In a prospective study conducted in Guinea
Bissau, the proportion of index cases was highest among children aged 2 to 9 years (24). The
lower seroprevalence in our study may be attributable to the younger study population,
which includes children 6 months to <5 years of age.
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The overall trend in tropical regions may suggest that the greatest rate of seroconversion
occurs in adolescence and adulthood (37), which is concerning as infection in older ages is
associated with increased rates of severe disease and complications (including skin
infections, pneumonia, and encephalitis) and mortality (13, 16, 38, 39). Furthermore, as
seroconversion more commonly occurs in adolescence or adulthood in tropical countries, a
large proportion of reproductive age women may remain susceptible to VZV in pregnancy.
VZV infection during pregnancy results in intrauterine infection in 25% of the cases (40).
Primary VZV infection in the first 20 weeks of gestation may result in congenital varicella
syndrome, in which infected mothers pass on the virus to the fetus (41). An estimated 2% of
fetuses are affected by the syndrome after maternal infection, resulting in outcomes ranging
from low birth weight to more severe anomalies such as chorioretinitis, microcephaly,
encephalitis, skin scarring and muscle wasting of the extremities severe anomalies (3, 41,
42). Infection during the third trimester and near delivery, in particular 5 days before or 2
days after birth, can cause severe and fatal infections in neonates (43). After delivery, in
utero infection is also associated with herpes zoster (shingles) in childhood (3). Therefore,
additional serologic studies in older age groups will be important to evaluate the burden of
VZV infection in the population and age-specific trends in virus circulation to identify
vulnerable subgroups.
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Moreover, the underlying reasons for varying response across provinces, in particular the
low seroprevalence observed in Maniema and Equateur and the high seroprevalence
observed in North and South Kivu, remain unclear. Such low rates of seroconversion may be
the result of reduced viral circulation in high ambient temperatures and humidity (44), or a
combination of other factors such as cross reactivity to other herpes viruses (45). In South
East Asia, outbreaks appear to be more common in cooler months and temperate regions
(15, 46–48), which is similar to the mountainous environment of North and South Kivu in
eastern DRC. However, it is also possible that other factors such as increased crowding in
homes in a cooler climate may favor transmission due to larger inoculum (16). Moreover,
political instability in the east has led to large groups of internally displaced persons and
while densely populated and crowded environments facilitate VZV transmission (49), the
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influx of potentially seronegative subjects may artificially decrease the prevalence in such
areas.
Additional limitations of our study include possible misclassification of serologic status.
However, compared to four commercially available kits, the multiplex assay exhibited high
validity for both sensitivity and specificity (sensitivity range: 89.5–100%; specificity range:
77.3–100%), making it unlikely that misclassification would significantly impact our
findings. Furthermore, the cost-effectiveness of the multiplex assay was high as we were
also able to evaluate the seroprevalence of antigens for mumps, measles, rubella, polio and
tetanus. Additionally, we initially categorized serologically indeterminate children as
positive; however, in sensitivity analyses, exclusion of these individuals did not change our
results. Furthermore, the younger children in the study population may still have maternal
antibodies
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There exists a safe and effective varicella vaccine, but it is only used in high income
countries (13). According to the WHO, varicella vaccine is not a high priority for routine
introduction into a national vaccination program (50). While the morbidity and mortality
associated with varicella is considerably lower than other VPDs, the virus still results in 140
million cases, 4.2 million hospitalizations, and 4,200 deaths annually in countries with
limited access to health infrastructure (13). Complications can lead to significant healthcare
and societal costs and cause a sizable burden for patients and caregivers; however, limited
information exists on the health burden of varicella in developing countries and tropical
climates where a higher proportion of cases may occur among adults. In these areas,
varicella morbidity and mortality may be higher than that of developed countries (51).
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Additional epidemiologic information regarding VZV infection is of particular interest in
DRC as the clinical presentation is similar to that of monkeypox virus (MPXV), a zoonosis
endemic to the Congo basin, where a steady increase in incidence has been reported since
the cessation of routine smallpox immunization in 1982 (52). Limited data available through
MPX surveillance in DRC provide evidence that varicella is circulating (53, 54) and findings
suggest VZV infection is commonly mistaken for MPX in central Africa (54–56).
To our knowledge, this is the first study to provide a national estimate of VZV exposure
among children. Evidence of the burden of VZV remains incomplete and additional
serologic studies in older populations will be required to better understand transmission and
its public health significance in the DRC.
Acknowledgments
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Funding Source: This work was supported by the Bill and Melinda Gates Foundation, Seattle, WA [grant number:
OPP106668].
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Figure 1.
Study inclusion for assessment of varicella-zoster virus seroprevalence among children 6–59
months of age, 2013–2014 Demographic and Health Survey, Democratic Republic of Congo
(unweighted)
a Those with missing seroprevalence data include: 18 who were not present, 129 who
refused participation, 120 for whom the DBS sample was not found in the database, 20 for
whom the barcode to match DBS sample was not known, and 31 “others”; b VZV stands for
varicella-zoster virus
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Figure 2.
Varicella-zoster seropositivity according to age (in months) among 6–59 months old 2013–
2014 DRC-DHS respondents (linear trend line in red).
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Figure 3.
Varicella-zoster seropositivity by province and age among 6–59 months old 2013–2014
DRC-DHS respondents.
a BCD stands for Bas Congo; KIN stands Kinshasa; BDD stands for Bandundu; KOC stands
for Kasai Occidental; KOR stands for Kasai Oriental; EQT stands for Equateur; ORL stands
for Orientale; MAN stands for Maniema; NKV stands for North Kivu; SKV stands for South
Kivu; KTG stands for Katanga
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Table 1
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Weighted demographic characteristics by varicella-zoster serosurvey result of 6–59 month old respondents of
the 2013–2014 DRC-DHS.
Negative
(n=6,435)
Positivea
(n=815)
n (%)
n (%)
798 (94)
51 (6)
Chi-square
p-value
Child age
6–11 monthsb
1 year
1,578 (93)
113 (7)
2 years
1,468 (88)
193 (12)
3 years
1,350 (87)
198 (13)
4 years
1,241 (83)
260 (17)
Male
3,224 (89)
410 (11)
Female
3,211 (89)
405 (11)
1
974 (92)
85 (8)
2
1,357 (92)
115 (8)
3
1,217 (88)
168 (12)
4
1,067 (89)
137 (11)
5 or more
1,821 (85)
310 (15)
≤20 years
1,209 (92)
101 (8)
21–25 years
1,699 (91)
175 (9)
26–30 years
1,644 (90)
190 (10)
31–35 years
1,040 (89)
133 (11)
906 (90)
98 (10)
No education
1,274 (87)
187 (13)
Primary
2,867 (91)
293 (9)
Secondary/higher
2,295 (87)
335 (13)
Poorest
1,503 (92)
129 (8)
Poorer
1,555 (92)
135 (8)
Middle
1,299 (88)
172 (12)
Richer
1,158 (85)
198 (15)
Richest
920 (84)
181 (16)
Kinshasa
415 (87)
65 (13)
Bandundu
1,062 (86)
170 (14)
Bas-Congo
271 (86)
46 (14)
1,032 (96)
47 (4)
<0.0001
Child sex
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0.8888
Children in householdc
<0.0001
Mother's age at birth
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>35 years
0.0298
Mother's highest level of education
<0.0001
Wealth indexd
<0.0001
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Province
Equateur
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Negative
(n=6,435)
Positivea
(n=815)
n (%)
n (%)
Kasai-Occidental
524 (93)
38 (7)
Kasai-Oriental
743 (93)
52 (6)
Katanga
646 (86)
102 (14)
Maniema
245 (96)
10 (4)
Nord-Kivu
508 (83)
104 (17)
Orientale
569 (92)
53 (8)
Sud-Kivu
421 (76)
130 (24)
Urban
1,852 (85)
315 (15)
Rural
4,583 (90)
500 (10)
Chi-square
p-value
<0.0001
Residence
a
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Includes those who tested positive and indeterminant on Dynex M2 multiplex for varicella-zoster virus.
b
c
<0.0001
Only children 6 months of age and older were invited to participate in the serosurvey.
Children in household is the sum of boys and girls that currently live in the household.
d
Wealth index is a composite measure of a household's cumulative living standard, calculated from household ownership of selected assets (such as
televisions and bicycles), materials used for housing construction, and types of water access and sanitation facilities. Using principal components
analysis, the DHS separates all interviewed households into five wealth quintiles.
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Table 2
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Weighted logistic regression of sociodemographic factors associated with varicella-zoster seropositivity of 6–
59 month old 2013–2014 DRC-DHS respondents.
ORcrude (95%CI)
ORadjusteda (95%CI)
ref
ref
Child age
6–11 monthsb
1 year
1.11 (0.79, 1.57)
1.51 (0.81, 1.63)
2 years
2.06 (1.49, 2.83)
2.12 (1.53, 2.94)
3 years
2.28 (1.66, 3.14)
2.41 (1.74, 3.35)
4 years
3.27 (2.39, 4.47)
3.19 (2.31, 4.41)
Children in householdc
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1
ref
ref
2
0.97 (0.72, 1.30)
0.77 (0.57, 1.05)
3
1.58 (1.20, 2.08)
1.20 (0.90, 1.60)
4
1.47 (1.10, 1.95)
1.15 (0.85, 1.55)
5 or more
1.95 (1.51, 2.51)
1.49 (1.14, 1.96)
Mother's highest level of education
No education
ref
ref
Primary
1.01 (0.83, 1.22)
0.71 (0.57, 0.87)
Secondary/higher
0.70 (0.59, 0.83)
0.87 (0.69, 1.11)
ref
ref
Wealth indexd
Poorest
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Poorer
1.01 (0.79, 1.30)
0.84 (0.64, 1.08)
Middle
1.55 (1.22, 1.97)
1.23 (0.95, 1.59)
Richer
1.99 (1.58, 2.52)
1.31 (1.00, 1.73)
Richest
2.30 (1.81, 2.93)
1.54 (1.06, 2.23)
Kinshasa
ref
ref
Bandundu
1.03 (0.76, 1.40)
1.95 (1.32, 2.89)
Bas-Congo
1.09 (0.72, 1.64)
1.80 (1.15, 2.84)
Equateur
0.29 (0.20, 0.43)
0.56 (0.35, 0.88)
Kasai-Occidental
0.46 (0.30, 0.70)
0.81 (0.50, 1.30)
Province
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Kasai-Oriental
0.45 (0.31, 0.66)
0.72 (0.47, 1.11)
Katanga
1.02 (0.73, 1.42)
1.48 (1.02, 2.14)
Maniema
0.26 (0.13, 0.52)
0.44 (0.21, 0.92)
Nord-Kivu
1.32 (0.94, 1.85)
1.97 (1.33, 2.92)
Orientale
0.60 (0.41, 0.88)
1.17 (0.76, 1.82)
Sud-Kivu
1.98 (1.43, 2.75)
3.41 (2.28, 5.10)
Urban
1.56 (1.34, 1.81)
1.46 (1.15, 1.85)
Rural
ref
ref
Residence
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a
Using backwards selection, only significant predictors (alpha ≤0.05) were retained in the final model; predictors in adjusted model include all
variables in table 2.
b
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c
Only children 6 months of age and older were invited to participate in the serosurvey.
Children in household is the sum of boys and girls that currently live in the household.
d
Wealth index is a composite measure of a household's cumulative living standard, calculated from household ownership of selected assets (such as
televisions and bicycles), materials used for housing construction, and types of water access and sanitation facilities. Using principal components
analysis, the DHS separates all interviewed households into five wealth quintiles.
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