By March 2023, three-quarters of Canadians were immune to infection with the SARS-CoV-2 virus that causes COVID-19, according to a study published Monday in the Journal of the Medical Association.
The Working Group on Immunity to COVID-19 (WGICT) thus estimated the number of people who developed specific antibodies to COVID-19, either through infection or vaccination.
To do this, researchers analyzed 900,000 blood samples from adults across the country.
Data were analyzed according to three time periods: pre-vaccine (March 2020 to November 2020), vaccine deployment (December 2020 to November 2021), and Omicron waves (December 2021 to March 2023).
The results show that in the first two phases of the epidemic, few people in Canada had traces of SARS-CoV-2 infection: less than 0.3% in May 2020 and only 9% in November 2021.
“Despite high vaccine coverage in Canada, rates of infection with the highly infectious Omicron variant increased rapidly,” said Dr. Illustrated by Bruce Mazer. University Health Center.
Six months after the Omicron variant in Canada, by mid-June 2022, the rate of population-acquired immunity to infection had increased to 47%, an average monthly increase of 6.4% per month. It was finally over 75% in March 2023.
“During the omicron phase, rates of infection-derived immunity increased most rapidly in younger age groups and in the western provinces of Manitoba, Saskatchewan, Alberta and British Columbia,” said study co-director Professor David Buckeridge.
In March 2023, about 80% of adults aged 18 to 25 showed evidence of previous infection. This figure compares with 75% of 25- to 39-year-olds, 70% of 40- to 59-year-olds and 60% of those 60 and older.
The rate of increase in infection-acquired seroprevalence “has decreased significantly since spring 2023, although it continues to increase, particularly among older adults.”
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