This is a summary, written by members of the CITF Secretariat, of:

Murphy TJ, Swail H, Jain J, Anderson M, Awadalla P, Behl L, Brown PE, Charlton CL,  Colwill K,  Drews SD, Gingras AC, Hinshaw D, Jha P, Kanji JN, Kirsh VA, Lang ALS, Langlois MA, Lee S, Lewin A, O’Brien SF, Pambrun C, Skead K, Stephens DA, Stein DR,  Tipples G, Van Caeseele PG, Evans TG, Oxlade O, Mazer BD, Buckeridge DL. 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

The results and/or conclusions contained in the research do not necessarily reflect the views of all CITF members.

A study funded and led by the CITF, published in CMAJ, found that as of March 2023, over 75% of people in Canada had antibodies due to a SARS-CoV-2 infection. This is in marked contrast to the findings for May 2020 when only 0.3% of people in Canada had infection-acquired antibodies. The study was led by Drs. David Buckeridge and Bruce Mazer (both CITF, McGill University and Research Institute of the McGill University Health Centre), in collaboration with CITF Secretariat members Tanya Murphy, Hanna Swail, Jaspreet Jain, Tim Evans, and Olivia Oxlade.

Data were aggregated from over 700,000 samples collected by seven research studies: 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, and Saskatchewan Health Authority. 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).

Estimates of seroprevalence by geographical region and age were also determined.

Key findings:

  • In May 2020, less than 0.3% of people in Canada had infection-acquired antibodies. By November 2021, 9% of Canadians had immunity to SARS-CoV-2 from an infection. After the arrival of Omicron, seroprevalence increased rapidly so that by March 15, 2023, 76% of Canadians had detectable antibodies from infection.
  • Anti-S seroprevalence (vaccine-induced and infection-acquired) was consistently high (98% or higher) from November 2021 (vaccine roll-out) through the Omicron variant waves.
  • Prior to the Omicron waves, infection-acquired seroprevalence differed little by age. But during the Omicron waves, seropositivity increased more quickly in the younger age groups (under 25 years). By mid-June 2022, infection-acquired seroprevalence in people under 25 years was 57%, compared to 51% for those 25-39 years, 40% for those 40-59 years, and 25% for those over 60 years.
  • By March 2023, seroprevalence was between 70% and 80% in all age groups, except in adults over 60 years in which 60% had infection-acquired antibodies.
  • The rapid rise in infection-acquired antibodies occurred across Canada and was most pronounced in younger age groups and in the Western provinces of Manitoba, Saskatchewan, Alberta, and British Columbia.

Most Canadians were infected with SARS-CoV-2 for the first time during the Omicron era, after previously being vaccinated. As a result, many Canadians now have hybrid immunity against SARS-CoV-2. However, the authors conclude that variations by age and geography, along with 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.