Epidemiology of provincial SARS-CoV-2 infections.

In Alberta, the first COVID-19 wave peaked around 24 April 2020 with a 7-day average nucleic acid test (NAAT) positivity of 5.80% (Fig. 1). The first samples for this study were collected between 1 and 5 June 2020, approximately 6weeks after the first wave. Daily case counts were low from June through September 2020, with NAAT positivity staying under 2% until 6 October 2020. Overall seropositivity over the time period studied was 1.33% (95% CI 1.25 to 1.40) (crude) and 1.75% (95% CI 1.74 to 1.76) (adjusted). Seroprevalence peaked in January 2021 (crude 3.63%; adjusted 4.63%) (Table 1).

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FIG 1

(A) Daily new cases and positivity rates of COVID-19 NAT results verified in Alberta with monthly serosurvey periods and major pandemic-related events. (B) Location of Alberta in Canada. Map taken from the public domain (https://en.m.wikipedia.org/wiki/File:Alberta-map.png).

COVID-19 cases began to rise in October, reaching a maximum 7-day average rate of 9.74% on 6 December 2020 and declining from this second wave in January 2021 (Fig. 1). The seropositivity rates increased in these months with age and sex standardized rates of 1.61% in November (95% CI: 1.60 to 1.62), 2.47% in December (95% CI: 2.46 to 2.49), and 4.63% in January 2021 (95% CI: 4.61 to 4.65) (Table 1).

Antibody positivity by sex and age.

There were no significant differences in seropositivity by sex over the combined survey periods (P=0.11) (Table 2) or in most months, except for December, when seroprevalence among males was 2.27% (95% CI: 1.87 to 2.67%) and that among females was 1.77% (95% CI: 1.46 to 2.09%), with a Pvalue of0.05 (Fig. 2A; Table S2).

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FIG 2

IgG positivity across the serosurvey periods by (A) sex and (B) age group, with 95% CI error bars.

TABLE 2

Overall COVID-19 seroprevalence of Alberta by sex, age groups, and geography, June 2020 to January 2021 (N=93,993)

Demographic	Survey size (N)	IgG positive count (nPOS)	% IgG positive (95% CI)
Sex
Male	41,135	572	1.39 (1.28–1.50)
Female	52,858	673	1.27 (1.18–1.37)
Unknown	2	0	0
Age group (y)
0–9	885	24	2.71 (1.64–3.78)
10–19	2,786	44	1.58 (1.12–2.04)
20–29	7,233	140	1.94 (1.62–2.25)
30–39	12,305	217	1.76 (1.53–2.00)
40–49	12,161	202	1.66 (1.43–1.89)
50–59	15,340	231	1.51 (1.31–1.70)
60–69	19,539	200	1.02 (0.88–1.16)
70 to 79	14,800	117	0.79 (0.65–0.93)
≥80	8,945	70	0.78 (0.60–0.97)
City/area
Calgary	25,479	310	1.22 (1.08–1.35)
Edmonton	22,260	361	1.62 (1.46–1.79)
Red Deer	2,471	16	0.65 (0.33–0.96)
Lethbridge	4,102	20	0.49 (0.27–0.70)
Medicine Hat	4,413	11	0.25 (0.10–0.40)
Grande Prairie	2,619	12	0.46 (0.20–0.72)
Brooks	1,170	124	10.6 (8.83–12.36)
Rural	12,107	247	2.04 (1.79–2.29)
Alberta, other	18,130	120	0.66 (0.54–0.78)

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Overall, the median age of patients positive for SARS-CoV-2 antibodies was 49years (IQR: 35 to 63) (Fig. 2; Table 1; Table S2). The youngest median age for seropositive patients occurred in July (41.5years, IQR: 32 to 55.8), and the oldest median age occurred in September (58years, IQR: 51.5 to 69). Over the entire survey period, children 0 to 9years old had the highest seropositivity (2.71%, 95% CI: 1.64 to 3.78%), and this seropositivity was significantly higher than that of all other age groups except for the 20- to 29-year-olds, who had the second highest overall seropositivity (1.94%, 95% CI: 1.62 to 2.25%). There were 202/885 (22.8%) children aged ≤2years of age, of which 5/202 (2.5%) were seropositive.

During the second wave from November 2020 to January 2021, seropositivity increased significantly for all age groups except among 0- to 9- and ≥80-year-olds. In January, the highest seropositivity was seen among patients 0 to 9years old (7.22%, 95% CI: 2.07 to 12.37%) followed by those 30 to 39years old (5.07%, 95% CI: 3.95 to 6.19%), while the seropositivity for those ≥80 years old was lowest at 1.67% (95% CI: 0.83 to 2.50%).

Antibody positivity by geography and socioeconomic demographics.

Compared to that of most other cities, Calgary had a higher seropositivity in June (3.53%, 95% CI: 2.20 to 4.86%) (Fig. S1; Table S3), which aligns with the city’s known high infection rate in Spring 2020. The seropositivity for Calgary decreased to below provincial average until September onwards, when it rose again in conjunction with the second wave. In Edmonton, seropositivity was below or near the provincial average until October, after which it increased to 5.95% (95% CI: 5.01 to 6.89%) in January compared to 3.63% (95% CI: 3.28 to 3.98%) for the province. The midsized cities of Red Deer, Lethbridge, Medicine Hat, and Grande Prairie all had seropositivities below 1% except for the increase in January to 3.42% (95% CI: 1.43 to 5.40%) for Red Deer. The small municipality of Brooks had a very high seropositivity rate of 19.8% (95% CI: 11.4 to 28.2%) in June, largely due to an outbreak at a large meat-packing plant 2 months earlier. Although the seropositivity in Brooks decreased over time, it consistently remained the highest in the province with a mean seroprevalence of 10.6% (95% CI: 8.83 to 12.36%) over the 8 study months compared to 1.21% (95% CI: 1.14 to 1.28%) for the rest of the province (Table 2). From August to January, seropositivity in rural areas was consistently above the provincial average and higher than that in most urban municipalities.

Overall, patients who were seropositive came from neighborhoods with median household incomes lower compared to those of seronegative individuals ($87,500, 95% CI: $85,600 to $89,500 compared to $97,500, 95% CI: $97,300 to $97,800, respectively; P<0.001) (Table S4). Dissemination areas (DAs) for seropositive patients had significantly lower marriage rates, larger household sizes, and higher proportions with education level at or below high school (all P<0.001) (Table S4). Among DAs for seropositive patients, the proportions of immigrants (27.2%, 95% CI: 26.3 to 28.2%) and visible minorities (30.9%, 95% CI: 29.5 to 32.4%) were significantly higher than those for seronegative patients (21.2%, 95% CI: 21.1 to 21.3% and 22.9%, 95% CI: 22.7 to 23.0%, respectively; both P<0.001).

Longevity of nucleocapsid antibodies.

Detection of SARS-CoV-2 nucleocapsid antibodies in patients who tested positive by NAAT increased in the first month following NAAT, reaching a maximum seropositivity of 89.9% (95% CI: 82.7 to 97.0%) on week 4 after positive NAAT (Fig. 3). After week 6, antibody detection decreased to 50.0% (95% CI: 19.0 to 81.0) on week 16. After week 20, large fluctuations in seropositivity were observed with large uncertainties due to sample sizes smaller than 10. However, a positive serology result was observed in a patient on week 41 (9 to 10months) after the patient’s last recorded positive NAAT result.

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FIG 3

(A) Counts of IgG status and (B) positivity rates for patients with positive NAAT results by number of weeks from NAAT to serology test (n=867). The number of patients with greater than 20weeks from positive NAAT result to serology test.

If the criteria for seropositivity were adjusted by omitting the spike protein confirmation and lowering the nucleocapsid antibody instrument cutoff from 1.4 to 0.7 (5,–7), the sensitivity of SARS-CoV-2 antibody testing to detect previously positive NAAT cases increased from 66.1% to 85.0% (Table S5). This same adjustment for positivity criteria did not have a great effect on concordance (97.9% versus 99.3% with the original 1.4 antibody with spike protein confirmation criteria). While the number of potential false-negative samples (NAAT-positive but serology-negative) decreased from 294 to 130 by dropping the cutoff, the number of potential false positives (NAAT-negative but serology-positive) increased from 179 to 539. Given the higher agreement of the 0.7 cutoff with NAAT testing, applying this looser positivity threshold would increase the population-adjusted positivity of Alberta in January 2021 from 4.63% (95% CI: 4.61 to 4.65%) to 7.24% (95% CI: 7.21 to 7.26%) (Fig. S2).

Sample collection.

From June 2020 to January 2021, residual blood specimens (serum, plasma, and heparinized plasma) were gathered monthly from clinical testing laboratories across Alberta and sent to the Public Health Laboratory (ProvLab) for SARS-CoV-2 serology testing. These samples were collected by phlebotomy from community patients for test requests unrelated to COVID-19 serology. For each monthly period, the larger laboratories in the two major Alberta cities, Calgary and Edmonton, contributed 1 day’s worth of blood samples (approximately 4,000 to 5,000 samples from each site). The monthly contributions from the laboratories at the Regional Hospitals in smaller cities of Grande Prairie, Red Deer, Lethbridge, Brooks, and Medicine Hat consisted of 4 to 7 consecutive days’ worth of blood samples (approximately 500 to 1,000 samples from each site). The wider time windows for these laboratories, which also serve as testing hubs for their surrounding rural areas, were chosen to increase representation from around the province.

SARS-CoV-2 nucleic acid testing.

As part of its public health strategy, Alberta offered SARS-CoV-2 molecular testing beginning in January 2020 for travel-related requests using a gel-based reverse transcriptase PCR (RT-PCR) assay (COVID-19-specific and pan-coronavirus) (4). From February 2020 onward, laboratory-developed real-time RT-PCR assays targeting the SARS-CoV-2 envelope (E) and/or RNA-dependent-RNA-polymerase (RdRp) were implemented, followed by a laboratory-developed E/RdRp/MS2 multiplex assay or Seegene (2019-nCoV assay, Seegene), Xpert (Xpress SARS-CoV-2, Cepheid), Aptima (SARS-CoV-2 assay, Hologic), BD Max (BioGX SARS-CoV-2, BD Molecular Diagnostics), Simplexa (COVID-19 direct test, DiaSorin), or cobas 6800 (SARS-CoV-2 test, Roche) testing (depending on laboratory testing location) for all symptomatic patients (27, 28). Between 29 May 2020 and 4 November 2020, broad asymptomatic testing was made available to all Albertans.

SARS-CoV-2 antibody testing.

All samples were tested using an orthogonal testing algorithm. Samples were screened by the Abbott Architect SARS-CoV-2 IgG antibody assay, which detects SARS-CoV-2 antibodies directed against the viral nucleocapsid protein. All positives were confirmed by the DiaSorin SARS-CoV-2 IgG antibody assay which detects antibodies directed against the S1 and S2 regions of the spike protein as validated previously in the laboratory (14). Samples that were positive by the screening assay but negative by the confirmation assay were considered negative.

Data and analysis.

Serology test results were matched by specimen ID numbers to patients’ demographic information, including sex, date of birth, postal code of residence, and sample collection date. Survey periods were limited to the following specimen collection date ranges: 1 to 5 June, 6 to 10 July, 4 to 11 August, 7 to 11 September, 5 to 9 October, 9 to 13 November, and 7 to 10 December 2020 and 11 to 14 January 2021. Results from duplicate specimens of the same survey period were excluded. The rollout of COVID-19 vaccination to health care workers and long-term care residents in Alberta began in December 2020; any patients who had received a vaccine prior to specimen collection were excluded from the analysis.

Crude seropositivity rates were calculated using the total number of samples positive for SARS-CoV-2 antibodies divided by the total number of samples submitted. IgG positivity rates were standardized to 2020 Alberta population estimates by sex and 5-year age groups from Statistics Canada (29). Ninety-five percent confidence intervals (95% CI) were calculated for each positivity rate using Wilson score interval for binomial proportions.

Postal codes were used to classify patients’ residence as the following. (i) Urban: mid-to-major cities (Calgary, Edmonton, Red Deer, Lethbridge, Medicine Hat, Grande Prairie, and Brooks); (ii) Alberta, other: other smaller municipalities; and (iii) rural: any postal code beginning with T0. Postal codes were also matched to patients’ census dissemination areas (DAs), which are geographic units that generally cover a residential population of 400 to 700, allowing serology results to be linked to neighborhood-level socioeconomic statistics from the 2016 Canadian census long-form questionnaire, which was conducted by Statistics Canada (30). These variables included median household income, average household size, marital status by proportions of married/common-law or single Albertans, proportions of highest education level achieved (below high school, high school, diploma/certificate, Bachelor’s degree, or graduate/professional degree), proportions by immigration status (immigrant, recent immigrant within the past 5 years, or nonpermanent resident), visible minority status (31), and group proportions (self-identify as Aboriginal or of African, Chinese, non-Chinese East Asian, South Asian, Latin American, or Middle Eastern descent). The population means of these summary measures were compared between the IgG positive and IgG negative groups using t tests.

Finally, previous SARS-CoV-2 nucleic acid testing (NAAT) results extracted from the ProvLab Laboratory Information System were merged with IgG results by patients’ Alberta personal health numbers (PHNs). Patients were then classified as (i) having no NAAT results at any time prior to phlebotomy for the serology tested specimen, (ii) having at least one positive NAAT result prior to serology testing, or (iii) having only negative or inconclusive NAAT results prior to serology testing. Only NAAT testing done prior to the date of the patient’s IgG sample was included in our analysis.

We thank all the clinical chemistry laboratories in the province involved in sending samples (both Alberta Precision Laboratories and Dynalife Medical Labs); without their help and support, this work would not have been possible. We also acknowledge the tremendous effort of the staff at the Public Health Laboratory for testing and storing samples and for collating the data for this large study.

We declare no competing interests with regard to this publication. All authors have completed an ICMJE form. The authors affirm that this article is an honest, accurate, and transparent account of the study being reported. No important aspects of this study have been omitted. Any discrepancies from this study as originally planned have been fully explained. This study was supported in part by Alberta Health and the Canadian Immunity Taskforce.

C.L.C., methodology, validation, data curation, writing – original draft, writing – review and editing, project administration; L.T.N., methodology, software, validation, formal analysis, data curation, writing – original draft, writing – review and editing, visualization; A.B., investigation, data curation, writing – review and editing; J.F., investigation, data curation, writing – review and editing; S.S.P., methodology, software, validation, formal analysis, data curation, writing – original draft, writing – review and editing, supervision, visualization, project administration; C.M., validation, resources, writing – review and editing; C.L., validation, resources, writing – review and editing; D.H., validation, resources, writing – review and editing; C.L., validation, resources, writing – review and editing; K.S., validation, resources, writing – review and editing; J.N.K., writing – original draft, writing – review and editing, visualization, project administration, supervision; N.Z., validation, resources, writing – review and editing; N.L., validation, writing – review and editing; M.M., conceptualization, resources, writing – review and editing, funding acquisition, supervision; G.T., conceptualization, methodology, validation, resources, writing – review and editing, supervision, project administration, funding acquisition.