75% of Canadians had COVID-19 immunity in March: Study

In the latest study on COVID-19 by the COVID-19 Immunity Task Force (CITF), it is believed that as of March 2023, three-quarters of Canadians had immunity against SARS-CoV-2.

The CITF team collected data using blood samples to estimate trends around the virus.

Using three time periods: pre-vaccination (March 2020 to November 2020); vaccine roll-out (December 2020 to November 2021); and the Omicron waves (December 2021 to March 2023), they were able to determine during the first two phases of the pandemic that very few Canadians had SARS-CoV-2 in their blood.

In May 2020, when COVID-19 was relatively new in Canada, less than 0.3 per cent of Canadians were showing a previous infection. By November 2021, that percentage rose to 9 per cent.

“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 — the level of a pathogen in a population, as measured in blood serum — had risen to 47 per cent, with an average monthly increase of 6.4 per cent per month. It ultimately reached over 75 per cent by March 2023.”

The CITF has been able to determine the rate of increase in infection-acquired seroprevalence has slowed significantly since Spring 2023, but notes it is still increasing.

The complete study done by the CITF has been published in the Canadian Medical Association Journal.

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