More than 75% of Canadians had immunity to SARS-CoV-2 due to infection by March 2023

MONTREAL, Aug. 14, 2023 (GLOBE NEWSWIRE) -- By March 2023, after 16 months dominated by the Omicron variant, three quarters of people in Canada had immunity due to infection against SARS-CoV-2, the virus that causes COVID-19. Today, the COVID-19 Immunity Task Force (CITF) published the most comprehensive peer-reviewed analysis of pan-Canadian seroprevalence estimates in CMAJ (Canadian Medical Association Journal).

The CITF, with help from seven collaborating teams of scientists, used aggregate data to estimate trends in both infection-acquired and vaccine-induced SARS-CoV-2 seroprevalence across Canada. Three time periods were analyzed: pre-vaccination (March 2020 to November 2020); vaccine roll-out (December 2020 to November 2021); and the Omicron waves (December 2021 to March 2023). All data were derived from blood samples collected from CITF-supported general population studies with data at multiple time points. Beyond the studies that contributed to this analysis, the CITF, which is financed by the Public Health Agency of Canada, has funded 120 research studies with a focus on immunity to SARS-CoV-2, many of which have engaged underrepresented populations and those living in long-term care homes.

During the first two phases of the pandemic, in contrast to the burden on Canadian society and health systems, few people in Canada had evidence in their blood of a previous SARS-CoV-2 infection: less than 0.3% by May 2020, and only 9% by November 2021. The low infection rates in Canada before Omicron were observed in other high-income countries in Europe and North America as well.

"Despite high vaccine coverage in Canada, the rate of infection rose rapidly with the highly contagious Omicron variant," explains Dr. Bruce Mazer, study co-lead, Associate Scientific Director, Strategy at the CITF and Senior Scientist at the Research Institute of the McGill University Health Centre. "After six months with the Omicron variant circulating in Canada, in mid-June 2022, infection-acquired seroprevalence had risen to 47%, with an average monthly increase of 6.4% per month. It ultimately reached over 75% by March 2023."

"During Omicron, rates of infection-acquired immunity increased faster in younger age groups and in the Western provinces of Manitoba, Saskatchewan, Alberta, and British Columbia," adds study co-lead Dr. David Buckeridge, Scientific Lead, Data Management & Analysis at the CITF and Professor, School of Population and Global Health, McGill University. "By March 2023, roughly 80% of adults aged 18-25 had evidence of a previous infection. That's compared to approximately 75% of those aged 25-39 years, 70% of those aged 40-59 years, and 60% of those 60 and over."

The rate of increase in infection-acquired seroprevalence has slowed significantly since Spring 2023, although it continues to rise, especially in older adults. However, the authors conclude that "variations by age and geography and the potential for waning antibody levels suggest that public health policy and clinical decisions will need to be tailored to local patterns of population immunity."

The analysis used 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.

The collaborating study teams were from: Action to beat Coronavirus Study (St. Michael's Hospital, a site of Unity Health Toronto), Alberta Precision Laboratories, Cadham Provincial Laboratory, Canadian Blood Services, CanPath, Héma-Québec, Mount Sinai/Lunenfeld-Tanenbaum Research Institute, Saskatchewan Health Authority, University of Ottawa. 

"The Evolution of SARS-CoV-2 Seroprevalence in Canada ? A Time-Series Study, 2020-2023". CMAJ 2023 August 14;195:E1030-7. doi: 10.1503/cmaj.230949

About the COVID-19 Immunity Task Force 

The Government of Canada established the COVID-19 Immunity Task Force (CITF) in late April 2020 to catalyze, support, fund, and harmonize research on SARS-CoV-2 immunity for federal, provincial, and territorial decision makers in their efforts to protect Canadians and minimize the impact of COVID-19. The Task Force's Secretariat works closely with a range of partners, including governments, public health agencies, institutions, health organizations, research teams, other task forces, engaging communities and stakeholders. To date, the CITF has supported 120 studies across Canada that have been generating critical insights on the levels, trends, nature, and duration of immunity arising from SARS-CoV-2 infection and COVID-19 vaccination.

See our latest seroprevalence estimates on our Seroprevalence in Canada page.

Media contact:

Caroline Phaneuf
Communications & Knowledge Translation, COVID-19 Immunity Task Force
[email protected]
+1.514.778.5092

The views expressed herein do not necessarily represent the views of the Public Health Agency of Canada.