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A new study involving researchers from the University of Toronto suggests that, by last spring, had developed antibodies in response to COVID-19 infections.

Using aggregate data from various collaborating pan-Canadian teams, researchers found that few people showed SARS-CoV-2 infection in their blood during the country’s pre-vaccination and vaccine rollout phases – 0.3 per cent and 9 per cent, respectively.

By mid-June 2022, however, infection rates jumped to 47 per cent and increased to 75 per cent by March 2023 as the highly transmissible and immune-evasive Omicron variant spread across the country. Infection appears to be slowing, though it continues to rise among older adults.

The findings – – used data from , which is based at U of T’s Dalla Lana School of Public Health.

“We aimed to untangle the relationship between vaccination status – including the number of doses, brands, and time between doses – and infection risk while accounting for prior infection, adherence to recommendations from public health officials and other factors which impact immunity such as age,” says study investigator Kimberly Skead, who is national scientific co-ordinator for CanPath and recently defended her PhD in molecular genetics at U of T and the Ontario Institute for Cancer Research.

Since 2020, CanPath, with other national partners, has been tasked by the Public Health Agency of Canada (PHAC) and  to support a national COVID-19 serological surveillance study. In March 2020, with $5.2 million in funding, the CanPath team began collecting COVID-19 data from more than100,000 Canadians, including dried blood spots at three different time points: pre-vaccination (March 2020 to November 2020); vaccine rollout (December 2020 to November 2021); and the Omicron waves (December 2021 to March 2023).

Following the health of its 330,000 participants over the long term, CanPath is well-positioned among CITF collaborators to provide vital insights into the nature and duration of immune response and the need for and timing of vaccine booster doses.

“CanPath’s extensive participant base has been instrumental in enabling us to find these differences in exposure and immunity across Canada,” says study investigator Philip Awadalla, who is national scientific director for CanPath and a professor at the Dalla Lana School of Public Health and the department of molecular genetics in the Temerty Faculty of Medicine.

“The results from our studies, combined with the wealth of data and ongoing research from our collaborators, can contribute to different vaccine responses as the pandemic progresses.”

Certain measures can be taken to ensure that our ongoing narrative remains one of resilience, adaptability, and safeguarding our collective well-being, the researchers say.

“Given the lingering risks of post-COVID conditions and the vulnerability of certain individuals to severe outcomes, a proactive approach is essential,” says Victoria Kirsh, a study investigator who is a scientific associate at and an assistant professor at the Dalla Lana School of Public Health.

“To safeguard against future variants, it’s important to continue monitoring COVID-19’s spread and roll out booster campaigns ahead of upcoming waves.”

Read more about the COVID-19 Immunity Task Force