COVID-19: 75% of Canadians were infected by virus by spring 2023

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A surge in infections from the Omicron variant last respiratory virus season left three quarters of Canada’s population with immunity against SARS-CoV-2, the virus that causes COVID-19.

The data on infection-fuelled immunity — as opposed to vaccine protection — was produced by the COVID-19 Immunity Task Force (CITF) and published Monday in the Canadian Medical Association Journal.

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It shows that by March of this year, after 16 months when Omicron was the dominant circulating strain, more than 75 per cent of people had been infected.

“Despite high vaccine coverage in Canada, the rate of infection rose rapidly with the highly contagious Omicron variant,” said Dr. Bruce Mazer, the study’s co-lead and a 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 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.”

That’s in stark contrast to earlier stages of the pandemic. During the first two phases, few people had evidence in their blood of a previous SARS-CoV-2 infection. There were less than 0.3 per cent infected by May 2020, and only nine per cent by November 2021. Those rates are in keeping with other high-income countries in North America and Europe.

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Omicron combined with looser restrictions sparked the latest wave, especially among younger people.

“During Omicron, rates of infection-acquired immunity increased faster in younger age groups and in the western provinces of Manitoba, Saskatchewan, Alberta and British Columbia,” said study co-lead Dr. David Buckeridge, a professor of global health at McGill.

“By March 2023, roughly 80 per cent of adults aged 18-25 had evidence of a previous infection,” said Buckeridge in a release Monday. “That’s compared to approximately 75 per cent of those aged 25-39 years, 70 per cent of those aged 40-59 years, and 60 per cent of those 60 and over.”

Rates of infection have slowed significantly since the spring, though it continues to rise, especially among older adults.

B.C., like many Canadian jurisdictions, plans to roll out vaccines tailored to the latest Omicron variant in the fall. The CITF study authors say the data on infection rates should be used to watch for “variations by age and geography and the potential for waning antibody levels” in determining future public health policy surrounding COVID-19.

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The CITF compiled the numbers along with seven collaborating teams of scientists. They used aggregate data to estimate trends in both infection-acquired and vaccine-induced SARS-CoV-2 presence in blood samples.

The numbers are from three distinct phases of the pandemic: pre-vaccination, from March to November 2020; during the rollout of vaccines, from December 2020 to November 2021; and during the Omicron waves, from December 2021 to March 2023.

The CITF, which is funded by the Public Health Agency of Canada, has supported 120 research studies focused on immunity to SARS-CoV-2, often among underrepresented populations and those living in long-term care.

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