A new study suggests more than one per cent of Torontonians had COVID-19 antibodies in their blood as of late spring, slightly higher than the overall Canadian population.

Canadian Blood Services and Canada’s COVID-19 Immunity Task Force (CITF) released its final results of a collaborative nine-province SARS-CoV-2 seroprevalence study on Tuesday, which analyzed more than 37,300 blood samples from across Canada, except for Quebec and the Territories.

Overall, fewer than one per cent of Canadians had antibodies for the virus based on the results.

The samples were collected between May 9 and June 18 and revealed that as few as 0.7 per cent of Canadian adult blood donors tested positive for antibodies to COVID-19.

“...These results once again tell us how few Canadians were infected by SARS-CoV-2 by the end of May,” Professor Catherine Hankins, CITF co-chair, said in a press release. “This shows that when all actors, especially individual citizens, follow good public health practices, the risk of infection diminishes considerably,” she added.

According to the report, antibodies are a key indicator of past infection from the virus and can generally be detected within two weeks.

blood samples

Of the 37,737 samples collected, slightly over half were from male donors and the majority of donors were between 40 to 59 years old.

More than 19,800 samples were taken from Ontario, followed by 5,644 from Alberta and 4,962 from British Columbia..

As per each region included in the study, the results pointed to low seroprevalence of SARS-CoV-2 in all provinces and cities across Canada.

The report noted that seroprevalence was slightly higher in Ontario compared with other provinces and there was slightly higher seroprevalence among females compared with males, but ultimately there was no difference between age groups.

“Whereas Prince Edward Island and New Brunswick have the lowest seroprevalence at 0% and 0.26% respectively, Ontario showed the highest, with 0.96%,” Sheila O’Brien, associate director, epidemiology and surveillance at Canadian Blood Services said in the release.

Although Quebec was not included in the study, the report noted that the province has the highest seroprevalence in the country to date (2.23 per cent) based on a recent study by Héma-Québec.

Of Canada’s major cities, Ottawa had the highest percentage of seroprevalence at 1.29 per cent, followed by Toronto with 1.07 per cent and Vancouver with 0.60 per cent.

Although the study suggests that a very low amount of Canadians were infected with the virus by the end of May, Hankins noted that the results could be an underrepresentation of how many Canadians actually possess COVID-19 antibodies.

“First, antibody levels do decline rapidly and may have disappeared in some people by the time of testing,” Hankins said. “Second, blood donors in general are health-conscious and healthier than the general population.’’

Researchers also noted that with COVID-19 cases increasing in some parts of the country, the study’s results point to the need for more testing.

“...Even if we doubled or tripled the estimates from this study, there is little to no likelihood that levels of immunity in the population are high enough to slow down a second wave of COVID-19 infection,” CITF Co-Chair Professor David Naylor said in the release. “As such, it’s critical to ramp-up testing and tracing capacity across the country to reduce risk in settings such as workplaces and schools, and to interrupt any chains of transmission quickly to prevent spread.”

The CITF was launched by the federal government in late April 2020 to track the spread of the virus across the country.

Canadian Blood Services is a not-for-profit charitable organization regulated by Health Canada, which provides blood, plasma and stem cell services for patients on behalf of all provincial and territorial governments, except Quebec.