TORONTO – 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.

About St. Michael’s Hospital

St. Michael’s Hospital provides compassionate care to all who enter its doors. The hospital also provides outstanding medical education to future health care professionals in more than 27 academic disciplines. Critical care and trauma, heart disease, neurosurgery, diabetes, cancer care, care of the homeless and global health are among the Hospital’s recognized areas of expertise. Through the Keenan Research Centre and the Li Ka Shing International Healthcare Education Centre, which make up the Li Ka Shing Knowledge Institute, research and education at St. Michael’s Hospital are recognized and make an impact around the world. Founded in 1892, the hospital is fully affiliated with the University of Toronto.

About Unity Health Toronto

Unity Health Toronto, comprised of Providence Healthcare, St. Joseph’s Health Centre and St. Michael’s Hospital, works to advance the health of everyone in our urban communities and beyond. Our health network serves patients, residents and clients across the full spectrum of care, spanning primary care, secondary community care, tertiary and quaternary care services to post-acute through rehabilitation, palliative care and long-term care, while investing in world-class research and education. For more information, visit

About the Dalla Lana School of Public Health

The Dalla Lana School of Public Health is a faculty of the University of Toronto, and Canada’s largest public health school. Our community is comprised of internationally recognized teachers, students, practitioners, policymakers and citizens.  We create new knowledge; educate changemakers; advance practice and guide the way to better, more equitable outcomes in population health and health systems. Join us at the forefront of change in health!

About the Angus Reid Institute

The Angus Reid Institute (ARI) was founded by pollster & sociologist, Dr. Angus Reid to enhance and encourage a better understanding of issues and trends affecting the public policy in Canada. Since 2014 The Institute has carried out over 500 studies on a vast array of topics ranging from foreign policy to poverty reduction. In the health sector Institute research has helped shape the debate on pharmacare, examined waiting times for elective procedures and provided up to date information on gaps in primary care and mental health services. Strictly nonpartisan, ARI is funded by the Reid family and partnerships with other not for profits. ARI has the status of a registered charity in Canada.

About the COVID-19 Immunity Task Force

The Government of Canada established the COVID-19 Immunity Task Force (CITF) in late April 2020 to catalyze, support, fund, and harmonize research on SARS-CoV-2 immunity for federal, provincial, and territorial decision-makers in their efforts to protect Canadians and minimize the impact of the COVID-19. To date, the CITF has supported over 100 studies across Canada that are generating critical insights on the levels, trends, nature, and duration of immunity arising from SARS-CoV-2 infection and COVID-19 vaccination. The CITF is overseen by a Leadership Group of volunteers that includes leading scientists and policymakers from across Canada. The Task Force and its Secretariat work closely with a range of partners, including governments, public health agencies, institutions, health organizations, research teams, other task forces, engaging communities and stakeholders.

About Sinai Health

Sinai Health is comprised of Mount Sinai Hospital, Hennick Bridgepoint Hospital, Lunenfeld-Tanenbaum Research Institute and its system partner Circle of Care. It delivers excellent care in hospital, community and home, focusing on the comprehensive needs of people. Sinai Health discovers and translates scientific breakthroughs, pushes boundaries for health solutions and educates future clinical and scientific leaders. Clinical areas of specialization include rehabilitation and complex continuing care, surgery and oncology, urgent and critical care, and women’s and infants’ health. Its Lunenfeld-Tanenbaum Research Institute ranks among the top ten biomedical research institutes in the world. Sinai Health is a full affiliate of the University of Toronto.

Media contacts:

Jennifer Stranges, Unity Health Toronto;

Dr. Angus Reid, Angus Reid Institute;

Rebecca Burns, COVID-19 Immunity Task Force;

Heidi Singer, Dalla Lana School of Public Health, University of Toronto;

The views expressed herein do not necessarily represent the views of the Public Health Agency of Canada.