Seroprevalence in Canada

In order to tailor public health strategies to the current pandemic context, it is important to understand the prevalence of COVID-19 across Canada. Since the winter of 2021-22 with the onset of the Omicron wave, PCR testing for diagnosis of SARS-CoV-2 has failed to keep up with the rapid growth in infection across the population. The CITF has therefore drawn on serosurveillance studies (measuring antibodies due to infection and vaccination in a person’s blood) to provide updated estimates of the magnitude and trends in SARS-CoV-2 infection in Canada.

The seroprevalence estimates presented below are based on data from over 20 studies that can be grouped into three categories: 1) blood donors from Canadian Blood Services and Héma-Québec; 2) anonymized discarded, or residual, blood samples from provincial laboratories; and 3) participants in CITF-funded research cohorts. All three sources contribute important information, but each source has different strengths and limitations.

Projects that have contributed information for this report include: Canadian Blood Services, Action to Beat Coronavirus, Alberta Precision Laboratories, BCCDC (funded by BCCDC), Canadian Antenatal Serosurvey, Canadian Partnership for Tomorrow’s Health, Statistics Canada’s CCAHS-1, Héma-Québec, Manitoba Seroprevalence, Public Health Ontario (funded by PHO), Saskatchewan Seroprevalence,  CHILD, CHU Ste-Justine serosurveillance study, EnCORE, SPRING, TargetKIDS!, Canadian Longitudinal Study on Aging, Prospective Evaluation of Immunity after COVID-19 vaccines in Seniors (PREVENT).

METHODS AND DATA

This analysis uses the presence of antibodies to the nucleocapsid protein as an indication of past infection and the presence of anti-spike antibodies to represent the overall seroprevalence representing both vaccine-induced and infection-acquired antibodies. Prior to vaccine roll-out in Jan 2021, the presence of anti-spike antibodies also indicates past infection. The seroprevalence estimates presented in this report are from three different sources: 1) Blood donors from Canadian Blood Services and Héma-Québec; 2) anonymized discarded or residual blood samples from provincial laboratories; and 3) participants in CITF-funded research cohorts. The data are assumed to provide an assessment of seroprevalence reflecting infection approximately 14 days or more earlier than their collection date, given the time it takes on average for infected individuals to develop measurable IgG antibodies in response to infection.

SOURCES OF DATA

Data were drawn from projects funded by the Government of Canada through its COVID-19 Immunity Task Force (CITF) and projects that have made their seroprevalence estimates publicly available. The funded projects and engaged partners reflect efforts by the CITF to assess seroprevalence of SARS-CoV-2 antibodies across Canada, as per its mandate beginning in April 2020. The types of projects collecting the data include provincial serosurveys and pan-Canadian studies of the general population, studies focusing on specific age-groups, special, and/or vulnerable subpopulations, and studies in COVID-19 “hotspots”, such as in occupational cohorts.

ASSAYS USED TO DETECT SARS-COV-2 ANTIBODIES

The measurement of antibodies against the spike (S), receptor binding domain (RBD), and nucleocapsid (N) proteins was performed by provincial laboratories, blood operators, and academic research laboratories. The assays used by provincial labs were Health Canada approved commercial ELISAs (Enzyme linked Immunosorbent Assay), including:

  • Anti-nucleocapsid IgG assay – Abbott Laboratories
  • Anti-nucleocapsid Total Ig antibodies – Roche Laboratories
  • Anti-spike Total Ig antibodies – Roche Laboratories
  • Anti-spike IgG antibodies – Diasorin
  • Anti-SARS-CoV-2 antibodies – MesoScale Discovery multiplex assay

Academic laboratories used both commercial assays and assays that they developed. These laboratory-developed assays used constructs of the SARS-CoV-2 proteins produced at the National Research Council Canada in Montreal or by other reputable, commercially available sources. Antibody detection was done via ELISA or high-throughput chemiluminescence assays. The development of global standards to help calibrate SARS-CoV-2 IgG 8 assays, Biological Arbitrary Units, has helped to decrease heterogeneity and facilitated comparison of and combination of results. For the nucleocapsid assays, the threshold for prior infection was at the manufacturer’s predetermined cut-off for the assay.

ANALYSIS

This report describes population estimates of SARS-CoV-2 seroprevalence measured across the course of the pandemic, within three distinct phases based on the predominant variant of concern at the time:

  • Pre-Delta: before August 1, 2021;
  • Delta: August 1 to December 14, 2021; and
  • Omicron: December 15, 2021, ongoing.

Anti-spike protein seropositivity can result from vaccination or infection. However, anti-nucleocapsid protein seropositivity only occurs following infection and does not occur following administration of any of the vaccines approved for use in Canada. Therefore, pre-vaccination (prior to Dec 15, 2020) evidence of infection-acquired antibodies includes anti-spike or anti-nucleocapsid seropositivity. After Dec 15, 2020, only anti-nucleocapsid seropositivity is taken as evidence of infection-acquired antibodies.

Seroprevalence results were summarized or presented by:

  • Geography: Canada and by province or region (no data are available for Canada’s three territories).
  • Average Age: Estimated from the age range of the participants

STATISTICAL MODEL FOR AVERAGE SEROPREVALENCE

Using a time-series approach with the aggregate-level seroprevalence estimates reported by each project, average seroprevalence over time was estimated using a Bayesian multivariate generalized linear model. The model, appropriate for count data where the proportion positive cannot exceed 100%, used a logit link with beta-binomial distributed errors. The predictors for anti-N seroprevalence included natural splines for the time predictor, and province-specific intercepts and coefficients with partial pooling. Hence, the time trend could vary by province while sharing information across provinces. The predictors for anti-S seroprevalence included natural splines for the time predictor and logit of anti-N seroprevalence. The 95% credible intervals were between the 2.5% and 97.5% quantiles of the Markov chain Monte Carlo (MCMC) samples.

LIMITATIONS

The serological assays used are heterogeneous, with different assays using different measurement units. By reporting the proportion of positive samples (seropositivity) we avoid the need to standardize different measurement units. However, it should be noted that different assays have different inherent characteristics (sensitivity, specificity, thresholds), which affect how positive samples are determined. Further, using the nucleocapsid antibody as the indication of prior infection has limitations. These limitations are due to factors such as antibody levels decay over time, low immunogenicity, the inability to know exactly when infection occurred, a lag of one- to two-weeks for maximal antibody generation, and the fact that it is not possible to differentiate between the first and recurrent infections. Consequently, the heterogeneity in estimates has increased during the Omicron era. In BC, for example, the increasing variation in seroprevalence estimates is due to including a broader age range in the source population. Given these limitations, estimates of seroprevalence should not generally be interpreted as direct measures of cumulative infections over the course of the pandemic. However, absolute changes in anti-N seroprevalence over short intervals (e.g., a few months) are likely to accurately represent sudden increases in infections over the interval.

How do serosurveys work?

Serosurveys determine the rate of seropositivity to SARS-CoV-2 in a given population by measuring the presence of antibodies against components of the virus in an individual’s blood. Infected individuals respond by making antibodies against multiple viral proteins, including the nucleocapsid and the spike proteins. In North America, all vaccines currently approved and in use are based on the spike protein and its receptor binding domain. Thus, when an individual has antibodies that recognize the nucleocapsid protein, it can be interpreted as a sign of past infection. In contrast, when an individual has antibodies that recognize the spike protein, it could be due to either vaccination or infection.

Results up to September 30, 2023 | Infection-acquired seroprevalence in Canada increases slightly since end of summer

Infection-acquired seroprevalence in Canada has shown a slight increase after being stable for several months. As at September 30, 2023, infection-acquired seroprevalence in Canada is 79.1%, up from 77.4% at the end of August.

Overall, infection-acquired seroprevalence increased significantly between November 2021, prior to Omicron being reported in Canada, and January 2023: rising from 5.4% (95% Credible Interval [Crl]: 4.5 to 6.4) to over 74% since Omicron became the dominant variant. In 2023, infection-acquired seroprevalence increased more slowly. Between January 1 and September 30, 2023, infection-acquired seroprevalence increased from 74.0% (95% Crl: 71.8 to 76.0) to 79.1% (95% CrI: 77.0 to 81.0), up 2% from August 31, 2023.

SARS-CoV-2 seroprevalence in Canada
(April 13, 2020 to September 30, 2023)

The following plot shows seroprevalence acquired from infection (Anti-nucleocapsid) and seroprevalence acquired from infection or induced from vaccination (Anti-spike) for all age groups, combined, by region over time.

Data notes:
Each point represents a seroprevalence estimate from a project at the mid-point of a sample collection period. The black line represents the estimated average seroprevalence weighted by sample size. The light grey bands represent the 95% credible confidence interval.

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The state of infection-induced seroprevalence in Canada

The interactive map below was produced by the CITF Data & Analysis team based on data from more than 20 CITF-funded studies or studies that have made their data publicly available. This map illustrates the percentage of Canadian adults with infection-acquired antibodies by the end of September 2023.

Seroprevalence by province over time

The findings from several seroprevalence studies across Canada showed that by the end of September in Canada’s regions:

  • Western Canada’s estimated mean seropositivity due to infection was 78.1% (95% CrI: 75.2 to 80.7) with little difference across the Western provinces, ranging from 78.0% (95% CrI: 75.0 to 80.4) in British Columbia to 81.7% (95% CrI: 79.2 to84.0) in Alberta.
  • Seropositivity was 78.2% (95% CrI: 75.6 to 80.7) in Ontario and 81.7% (95% CrI: 78.5 to 84.6) in Quebec.
  • Infection-acquired seroprevalence in Atlantic Canada was 70.1% (95% CrI: 66.4 to 73.3).

Antibody measured

anti-N estimate anti-S estimate

British Columbia

Alberta

Saskatchewan

Manitoba

Ontario

Quebec

Nova Scotia

Prince Edward Island

Newfoundland

New Brunswick

Antibody measured

anti-N estimate anti-S estimate

British Columbia

Alberta

Saskatchewan

Manitoba

Ontario

Quebec

Nova Scotia

Prince Edward Island

Newfoundland

New Brunswick

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Infection-acquired and vaccine-induced seroprevalence in the Northwest Territories

Results from sero-surveys carried out by Northwest Territories Public Health
March 2022 to January 2023

A cross-sectional serological survey funded by the CITF and done by Northwest Territories Public Health has estimated the seroprevalence of SARS-CoV-2 from vaccination and infection among NWT residents, breaking the results down by age group and region.

  • Seroprevalence due to vaccination or infection was over 95% in the Northwest Territories by January 2023.
  • Seroprevalence due to infection rose significantly in all age groups. In spring 2022, 47.3% of residents had antibodies due to infection (95% CI: 42.4 to 52.1) and by January 2023, 79.5% of residents had infection-acquired antibodies (95% CI: 76.9 to 81.9).
  • The data suggest children and young adults experienced a rise in infection-acquired seroprevalence earlier in 2022 than did older adults.
  • Overall, seroprevalence rose over time, however, due to the limited sample size in children, especially children under 12 years, and young adults, the trend over time is less clear. Adults over age 60 maintained a lower infection rate throughout 2022 as seen in Canadian blood donors from other provinces.

Infection-acquired seroprevalence in the Northwest Territories:
anti-nucleocapsid seropositivity estimates by age group and sampling period

The mean age was 49 years and 95% of participants fell within an age range of 12- to 83-years-old.

Seroprevalence due to infection by age in Canada

Results from sero-surveys carried out by Northwest Territories Public Health
March 2022 to January 2023

A cross-sectional serological survey funded by the CITF and done by Northwest Territories Public Health has estimated the seroprevalence of SARS-CoV-2 from vaccination and infection among NWT residents, breaking the results down by age group and region.

  • Seroprevalence due to vaccination or infection was over 95% in the Northwest Territories by January 2023.
  • Seroprevalence due to infection rose significantly in all age groups. In spring 2022, 47.3% of residents had antibodies due to infection (95% CI: 42.4 to 52.1) and by January 2023, 79.5% of residents had infection-acquired antibodies (95% CI: 76.9 to 81.9).
  • The data suggest children and young adults experienced a rise in infection-acquired seroprevalence earlier in 2022 than did older adults.
  • Overall, seroprevalence rose over time, however, due to the limited sample size in children, especially children under 12 years, and young adults, the trend over time is less clear. Adults over age 60 maintained a lower infection rate throughout 2022 as seen in Canadian blood donors from other provinces.

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Data from Canadian Blood Services and Héma-Québec only

Infection-acquired seroprevalence over time in Canada: a detailed interactive graph

This graph amalgamates Canadian Blood Services data since the beginning of the pandemic. It allows you to view the rates of infection-acquired seropositivity according to material deprivation, racial group, and age, providing insight about the characteristics of who has been most adversely affected by COVID-19. Such data can assist officials and policy makers when they are deploying resources and can help address issues regarding access to public health, disseminating information, and ensuring equity for all Canadians.

Please click on the red buttons at the top, “material deprivation”, “racial group”, or “age” to filter the results. You can also double click on the legend to isolate a trace.

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Seroprevalence by socioeconomic status over time

Canada (excluding Quebec and the Territories)

By the end of September, according to Canadian Blood Services, the most materially deprived donors continued to have higher infection-acquired seroprevalence (81.5%) than the least materially deprived individuals (79.5%).

1 As measured by the Material Deprivation Index, which makes use of postal code data.


1 As measured by the Material Deprivation Index, which makes use of postal code data.

SARS-CoV-2 antibodies

Infection (Roche assay) Vaccine only (Roche assay)

Data notes:

  • The bar height shows the estimated proportion of the subgroup who had antibodies to SARS-CoV-2, adjusted for assay sensitivity and specificity and standardized to the Canadian population. Error bars show 95% confidence intervals.
  • Antibodies from vaccine only are in red = anti-S IgG positive and anti-N IgG negative.
  • Antibodies from infection are in blue = positive for anti-nucleocapsid IgG.
  • The Material Deprivation index is an indicator for the lack of goods and services (deprivation) in a participant’s neighbourhood, based on the first three digits of their postal code (FSA). The index was developed was Statistics Canada using data from the 2016 Canadian Census on household income, unemployment rate, and high school education rate.
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Seroprevalence by racial group over time

Canadian Blood Services data show that self-identified racialized Canadians continued to have higher seroprevalence due to infection than Canadians who self-identified as white, but the rate of increase for both groups in the Omicron wave appeared similar.

Seroprevalence by race, month, and source of antibodies for Canada (excluding Quebec and the Territories)

SARS-CoV-2 antibodies

Infection (Roche assay) Vaccine only (Roche assay)

Data notes:

  • The bar height shows the estimated proportion of the subgroup who had antibodies to SARS-CoV-2, adjusted for assay sensitivity and specificity and standardized to the Canadian population. Error bars show 95% confidence intervals.
  • Antibodies from vaccine only are in red = anti-S IgG positive and anti-N IgG negative.
  • Antibodies from infection are in blue = positive for anti-nucleocapsid IgG.
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