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

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In an interview posted on the RI-MUHC website, Bruce Mazer, MD, discusses the COVID-19 Immunity Task Force (CITF)’s work in following the evolution of SARS CoV-2, or the virus that causes COVID-19, in Canadians since the onset of the virus. Three specific time periods were analyzed: pre-vaccination, vaccination roll-out, and the Omicron wave. Seven teams of collaborating scientists assisted in the collection and analysis of data.

What was discovered is that the earlier two phases of the pandemic resulted in less than 0.3% (by May 2020) and only 9% (by November 2021) infection-acquired seroprevalence in Canadians, despite the huge impact on our society and healthcare system. Yet, despite a much higher rate of vaccination for the Omicron wave (December 2021 to March 2023), the rate of infection increased significantly, such that:

“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.”

Bruce Mazer

By March 2023, younger cohorts (ages 18-25) were showing higher evidence of a previous infection, which diminished as the age of the cohorts increased (people aged between 25-39 years, 40-59 and over 60). Since Spring 2023, the rate of infection in older adults has increased, although the overall rate of infection-acquired seroprevalence has diminished.

Dr. Bruce Mazer is study co-lead, Associate Scientific Director, Strategy at the CITF and Senior Scientist at the Research Institute of the McGill University Health Centre.

Read the RI-MUHC Article

More than 75% of Canadians had immunity to SARS-CoV-2 due to infection by March 2023 August 14, 2023 RI-MUHC News.

Read the CITF publication in CMAJ

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