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TORONTO — The number of Canadian adults infected with COVID-19 has tripled during the fifth wave of the pandemic compared to the total number of adults infected in the previous four waves, according to a new study led by Toronto researchers.

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More than 5,000 Canadian adults – members of the Angus Reid Forum, a public survey cohort – participated in the fourth phase of the Action to Beat Coronavirus (Ab-C) study. The results of the study were published as a letter to the editor in the New England Journal of Medicine on Wednesday.

The adult participants performed a self-administered dried blood test between January 15 and March 15, 2022 and sent the blood samples back to the researchers for analysis. The research team then tested the samples for antibodies related to COVID-19.

Using these results, the researchers found that nearly 30 percent of Canadian adults were infected during the first wave of Omicron infections, compared to about 10 percent who had been infected in the previous four waves.

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Of those fifth-wave infections, one million were among the country’s 2.3 million unvaccinated adults — accounting for 40 percent of all unvaccinated adults, the study finds.

Patrick Brown, lead author of the Ab-C study and a biostatistician at the Center for Global Health Research at St. Michael’s Hospital, said the study was intended to paint a “complete and representative picture” of COVID-19 in the country given the lack of widely used PCR testing and COVID-19 data tracking.

“It’s pretty important for us to understand COVID in the community,” he said in a phone interview on Wednesday.

“The testing data is incomplete and we have essentially mostly stopped PCR testing in Canada, or at least Ontario. So it’s very important to have a representative sample of people getting these test kits to find out how much COVID there was and how much immunity we have in the population.”

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The study also found that antibody levels were much lower in adults who received only two doses of a COVID-19 vaccine than those who received three doses, meaning those who received three doses had stronger immunity to the virus.

And among the unvaccinated population — including those who had COVID-19 infection — their antibody levels were “fairly” lower than in people with three doses of the vaccine, Brown noted.

“(In) Canada we’ve had a little less COVID-19 than some other countries, especially the US. We have less natural protection and we really rely on vaccines in Canada to build immunity in our population,” he said.

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“Certainly three doses plus one infection was the maximum protection, but three doses of vaccine certainly gave very good protection – a great improvement over two doses alone.”

The Ab-C study is a collaboration between Unity Health Toronto, the University of Toronto’s Dalla Lana School of Public Health, the Angus Reid Institute and the Lunenfeld-Tanenbaum Research Institute at Sinai Health. It is funded by the federal government through its COVID-19 Immunity Task Force.

The research team has been tracking the pandemic in Canada since May 2020 with regular lived experience surveys and blood sampling.

Brown said the next phase of the study is already underway. The team began interviewing approximately 1,300 Canadian adults not infected with the original Omicron variant called BA.1 to determine if they were infected with the Omicron subvariant called BA.2 from March to June 2022.

“We are now preparing test kits to send out to our panel of people that we have come back to several times and this will be the fifth round of testing that we are sending them to better understand Omicron’s second wave,” he said.

“We found that the number of cases reported by public health is not as high as in the previous wave, the number of hospital admissions has not increased very much, but there have been a lot of infections … so we expect that in the whole population there has been quite a bit of COVID.”