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, Canadian Longitudinal Study on Aging, Prospective Evaluation of Immunity after COVID-19 vaccines in Seniors (PREVENT).

The data presented here are heterogeneous, with different measurement units used by different assays. 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 related to the lag-time between infection and detectable antibodies, antibody waning, and the inability to differentiate between first and recurrent infections.  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 July 31, 2022  | Percentage of Canadians with previous infection nears 60%

Overall, infection-acquired seroprevalence in Canada increased significantly between August 2021 and July 31, 2022: from 5.1% (95% confidence interval [CI]: 4.3–6.0) in the pre-Delta wave to 59.1% (95% CI: 53.5–66.85) by the end of July – after seven months with circulating Omicron variants.
We estimate this rise in seroprevalence during the Omicron phase of the pandemic corresponds to at least 18.2 million (95% CI: 16.5-20.5) Canadians being infected between December 15, 2021, and July 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 seven-month period is equivalent to more than 86,000 infections per day, which is more than 10 times the number of daily cases seen during the peaks of previous SARS-CoV-2 waves.

SARS-CoV-2 infection-acquired seroprevalence in Canada
(April 13, 2020 to July 31, 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.

The state of infection-induced seroprevalence in Canada

The interactive map below was produced by the CITF Data & Analysis team based on data from 22 CITF-funded or partner studies. This map illustrates the percentage of Canadian adults with infection-acquired antibodies by the end of July 2022.

Seroprevalence by province over time

Seroprevalence due to infection continued to increase during the Omicron wave across all Canadian provinces between December 15, 2021, and July 31, 2022. The findings from several seroprevalence studies across Canada showed:

  • In the last week of July, Western Canada’s estimated seropositivity due to infection ranged from 55.0% (95% CI: 49.0-60.9) in British Columbia to 67.4% (95% CI: 62.2-73.0) in Alberta.
  • Estimates in other provinces also increased over the same period, with seropositivity rising to 52.8% (95% CI: 48.8-57.1) in Ontario and 68.2% (95% CI: 54.0-92.4) in Quebec by the end of July.
  • Atlantic Canada had maintained the lowest seropositivity due to infection in Canada, however the increase in the spring of 2022 has positioned Atlantic Canada on par with other jurisdictions at just over 50% (Figure 3) seropositivity due to infection. The rate of this increase has been much higher than in other jurisdictions during the Omicron phase.

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

Seroprevalence due to infection by age in Canada

Conversion to infection-acquired (anti-N) seropositivity during Omicron increased in all age groups. The highest levels of seropositivity due to infection were observed in young adults (17–24 years), with about 73% seropositivity in the last week of July. Estimates of seropositivity due to infection decreased with increasing age in the last week of July: 25–39 years (68%), 40–59 years (56%), and 60+ years (40%).

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.

Seroprevalence by socioeconomic status over time

Canada (excluding Quebec and the Territories)

The data highlight the disparities in the burden of infection between the most materially deprived (Q5) and the least materially deprived (Q1)1 population groups – a phenomenon observed throughout the pandemic. The disparity grew during the month of July: 57.9% of the most materially deprived Canadian blood donors had infection-acquired antibodies, while 53.3% of the least materially deprived blood donors had evidence of COVID-19. This compares to 51.1% vs. 47.9%, respectively, in June.

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.

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.

Quebec

The proportion of donors with infection-acquired antibodies decreased according to age, from younger to older adults: 40.3% of donors 18 to 24 years old, 38.1% of donors aged 25-39 years old, 27.9% of donors aged 40-59 years old, and 18.7% of donors aged 60 and older (data for mid-March). Donors aged 60 and older experienced the largest increase in infection-acquired seropositivity over time across all age categories; a 6.6 time increase from 2.8% in January to 18.7% by mid-March.

SARS-CoV-2 antibodies

Infection (Roche assay) Vaccine only (Roche assay)

Data notes:

  • Quebec data gathered and analyzed by Héma-Québec used an ELISA assay developed by Héma-Québec. The Héma-Québec assay detects antibodies from either infection or vaccination (detects antibodies against the Receptor Binding Domain (RBD) of SARS-CoV-2).
  • Vaccination against SARS-CoV-2 began in December 2020 in Quebec. For each donor included in the study, Héma-Québec obtained their vaccination status, including their vaccination dates and the type of vaccine product, from the provincial vaccination registry named SI-PMI, managed by Quebec’s Health and Social Services Ministry.
  • The Héma-Québec seroprevalence study tested samples obtained between January and March, 2022.

Seroprevalence by racial group over time

The Canadian Blood Services data emphasize the persistent inequities in infection burden among racialized communities, which continued to have higher seroprevalence than self-declared white donors (62.3% vs 52%) at the end of July.

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.

Canadian Blood Services
and Héma-Québec reports

July 2022
June 2022
May 2022
Mid-May 2022
April 2022
Mid-April 2022
March 2022 (Héma-Québec)
March 2022
February 2022
January 2022
December 2021

CITF summaries of Canadian Blood Services
and Héma-Québec reports

July 2022
June 2022
(Héma-Québec)
June 2022
May 2022
Mid-May 2022
April 2022
mid-April 2022
March 2022
Mid-March 2022 (Héma-Québec)
Mid-March 2022
February 2022
Mid-February 2022
Mid-January 2022
December 2021
November 2021
October 2021
September 2021
August 2021
July 2021
June 2021
May 2021
January 2021
Oct/Nov 2020
May/June 2020