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Omicron triples Canada’s COVID-​19 infection count, study shows

May 19/2022

Nearly 30 per cent of Canadian adults – 9 million people – were infected during the Omicron variant wave early in 2022, compared with just 10 per cent who had been infected in the previous four waves, according to a new study led by Toronto researchers.

Despite the high numbers of infections, the study also revealed that every dose of vaccine and previous infection boosted immune responses. Canadian adults with three vaccine doses and a past infection from COVID-19 had the highest protection.

The findings, published in a letter to the editor in The New England Journal of Medicine, fill a gap in understanding the scale of COVID-19’s spread during the fifth wave, as well as Canadians’ immunity to the virus, either through vaccination or natural infection. Provinces scaled back COVID-19 molecular diagnostic testing in December 2021, leaving policymakers and the public without reliable data to inform pandemic responses and to gauge community risk.

“The incidence of Omicron variants, which rose worldwide from December 2021 even among vaccinated people, is poorly understood. This study quantifies SARS-CoV-2 incidence during the initial Omicron wave among Canadian adults, and the contribution of prior infection and vaccination to age-specific active immunity,” said Dr. Prabhat Jha, principal investigator of the Action to Beat Coronavirus (Ab-C) study and director of the Centre for Global Health Research at St. Michael’s Hospital of Unity Health Toronto. Dr. Jha is also a professor of epidemiology at the University of Toronto’s Dalla Lana School of Public Health (DLSPH).

The study analyzed more than 5,000 blood samples representative of Canadian adults – members of the Angus Reid Forum, a public polling cohort – from January 15 to March 15, 2022. From those results, the researchers determined that an estimated 9 million of 29.7 million Canadian adults were newly infected during the Omicron wave. Of those infections, one million were among the country’s 2.3 million unvaccinated adult population – representing 40 per cent of all unvaccinated adults.

The Ab-C study is a collaboration among Unity Health Toronto, DLSPH, the Angus Reid Institute, and the Lunenfeld-Tanenbaum Research Institute at Sinai Health. It is funded by the Government of Canada through its COVID-19 Immunity Task Force (CITF). Ab-C has been tracking the pandemic in Canada with periodic polling about lived experience and blood sample collection since May 2020, and will continue as long as the COVID-19 pandemic continues to evolve.

“Canada has kept natural infection levels generally low – perhaps less than 10 per cent of the adult population prior to Omicron – in contrast to many parts of the United States and England. So Canada must rely on vaccination – especially three doses for the older population. However, the proportion of adults vaccinated with third doses is still lower than ideal,” added lead author Dr. Patrick Brown, a biostatistician at the Centre for Global Health Research and the University of Toronto.

“If we take into account the fact that pediatric surveys have estimated that the proportion of infections among children was as high or higher than it was among adults and that new subvariants of Omicron continue to infect Canadians in the ongoing sixth wave, there are now millions more infections to add to the Ab-C study’s total,” states Catherine Hankins, Co-Chair of the COVID-19 Immunity Task Force. “In short, a substantial portion of the Canadian population now has hybrid immunity – defined as a combination of a past COVID-19 infection along with between one and three doses of a COVID-19 vaccine.”

The Ab-C study has started surveying approximately 1,300 adults who were not infected from the initial Omicron variant (called BA.1/1.1) to determine whether they were infected by the latest Omicron variant (called BA.2) from March to June 2022.

“We owe a great debt of gratitude to the thousands of Canadians, drawn from every region who took the time to share specimens of their blood and complete related surveys. Their participation made this study possible,” said Dr. Angus Reid, chairman of the Angus Reid Institute.

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