75 per cent of Canadians were infected with COVID-19 by March 2023, study finds

A recent study finds that 75 per cent of the Canadian population had detectable antibodies from COVID-19 through infections by March 2023.

Researchers from the COVID-19 Immunity Task Force (CITF), in collaboration with seven research studies, shared the findings in a study published in the Canadian Medical Association journal on Monday.

Researchers studied the Canadian population’s trends in infection and vaccination in three phases: prevaccination (March to Nov. 2020), vaccine roll-out (Dec. 2020 to Nov. 21), and the arrival of the Omicron variant (Dec. 2021 to March 2023). They also measured infection rates by geographical region and age using a time-series approach and data from more than 900,000 individual samples from 10 provinces.

According to the study, in the first two phases, seroprevalence -- the number of people in a population who test positive for a specific disease -- from infection was low, with less than 0.3 per cent of the Canadian population exposed to the virus in July 2020 and soon reaching nine per cent in Nov. 2021.

• Top health headlines, all in one place

• COVID-19 Brief newsletter: Sign up for fresh insights and analysis on the pandemic

Researchers said that after Omicron, infection rates increased reaching 75 per cent by March 2023.

"Despite high vaccine coverage in Canada, all previous increases in seroprevalence due to infection were dwarfed by the increase caused by the Omicron variant," said CITF associate scientific director and study lead Dr. Bruce Mazer in a news release Monday.

"After 6 months of the Omicron variant circulating in Canada, infection-acquired seroprevalence had risen to 47 per cent by mid-June 2022, with an average monthly increase of 6.4 per cent per month between December 15, 2021, and July 2022," added Mazer.

Researchers shared that despite vaccine coverage reaching 79 per cent by fall 2021, and cumulative reported cases remained at a low of 4.7 per cent, the arrival of the Omicron variant was not slowed down by high vaccine coverage and the population’s immunity from infections in earlier phases of the pandemic.

The study also revealed that rates of infection-acquired immunity increased faster in younger age groups, with close to 80 per cent adults under age 25 testing positive, 75 per cent in ages 25 to 39, 70 per cent in ages 40 to 59, and 60 per cent in those aged 60 and over by spring 2023.

The study concluded that "many people in Canada have hybrid immunity against SARS-CoV-2, but 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."