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).
  • Median Age: Estimated from the age range collected from each study

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, logit of anti-N seroprevalence, and the number of vaccination doses administered in the last six months per capita by province. 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. In Alberta, the reason for the increased variation in estimates is not currently known, but we are working with the Alberta Precision Labs scientists who contributed estimates from residual blood samples to understand possible reasons. 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 November 30, 2022 | Percentage of Canadians with infection-acquired antibodies exceeds 73%

Infection-acquired seroprevalence in Canada increased significantly between August 2021 and November 30, 2022: from 4.3% (95% credible interval [CrI]: 3.3 to 5.4) in the pre-Delta wave to 73.6% (95% CrI: 70.6 to 76.6) by the end of November 2022 – after 11 months with circulating Omicron variants.

We estimate this rise in seroprevalence during the Omicron phase of the pandemic corresponds to at least 26 million (95% CrI: 25.2 to 27.5) Canadians being infected between December 1, 2021, and November 15, 2022. The actual number of newly infected (or reinfected) Canadians may have been higher because some people infected early in the Omicron phase of the pandemic may no longer have detectable anti-N antibodies. The infection rate over this eleven-month period is equivalent to about 75,000 infections per day.

SARS-CoV-2 infection-acquired seroprevalence in Canada
(April 13, 2020 to November 15, 2022)

Anti-nucleocapsid seropositivity for all age groups, combined, by region

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 dark and light grey bands represent the 50% and 95% bootstrap confidence intervals, respectively.

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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 about 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 November 2022.

Seroprevalence by province over time

Seroprevalence due to infection during the Omicron waves, between December 15, 2021, and November 30, 2022, continued to increase across all provinces of Canada. The findings from several seroprevalence studies across Canada showed that in the last week of November:

  • Western Canada’s estimated mean seropositivity due to infection was 75.6% (95% CrI: 72.8 to 78.4) with little difference across the Western provinces, ranging from 74.5% in Saskatchewan to 76.8% in Manitoba.
  • Seropositivity rose slightly to 71.3% (95% CrI: 66.8 to 75.6) in Ontario. Seropositivity in Quebec was estimated to have risen to 76.3% (95% CrI: 67.9 to 85.4).
  • Although Atlantic Canada had maintained the lowest seropositivity due to infection in Canada through most of the pandemic, the increases since the spring of 2022 have resulted in a seroprevalence in Atlantic Canada comparable to other regions at 68.4% (95% CrI: 64.9 to 72.0) seropositivity due to infection. This is due to a rate of increase in Atlantic Canada during the early Omicron waves that was much higher than in other jurisdictions.

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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Seroprevalence due to infection by age in Canada

Conversion to infection-acquired (anti-N) seropositivity during Omicron increased in all age groups. In blood donors, the highest levels of seropositivity due to infection were observed in young adults (17–24 years), with approximately 83% seropositivity in the last week of November. Estimates of seropositivity due to infection decreased with increasing age in the last week of November: 25–39 years (76%), 40–59 years (72%), and 60+ years (57%).

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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)

The most materially deprived populations continued to have higher seroprevalence than the least materially deprived populations– 73.1% versus 69.4%, respectively, as of November 30, 2022.

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 emphasize the persistent inequities in infection burden among racialized communities. Racialized groups continued to have a higher seroprevalence rate (78.7%) than self-declared white donors (68.6%) as of November 30, 2022.

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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Looking for more of this data?

  • Do you need the population-level data shown in the graphs above? Try our download button above and get the data in an Excel file.
  • Do you need individual-level data? You will need to apply via the CITF Data Access Office once it is open in November 2022.
  • Do you need other population-level SARS-CoV-2 serology tabulations? Please contact our Data team at data@covid19immunitytaskforce.ca.

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and Héma-Québec reports

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