Few Canadian blood donors had coronavirus antibodies as of May: study

TORONTO — Less than one per cent of Canadian blood donors have tested positive for SARS-CoV-2 antibodies, which are present in those who have had coronavirus, suggesting there has been low exposure to the virus.

That’s according to a collaborative study by Canadian Blood Services and Canada’s COVID-19 Immunity Task Force (CITF), which analyzed 37,373 blood samples collected from nine provinces, excluding Quebec and the territories, between May 9 and June 18.

According to the researchers, antibodies are a key indicator of past infection and can generally be detected within two weeks of the onset of infection, which means that by the end of May, only 0.7 per cent of Canadian donors had been exposed to SARS-CoV-2.

Catherine Hankins, a co-chair of CITF, acknowledged the results of their study could underestimate the true seroprevalence in the population because antibodies rapidly decline and may have disappeared by the time of testing in some cases, and also because blood donors tend to be healthier than the general population.

“Nonetheless, these results once again tell us how few Canadians were infected by SARS-CoV-2 by the end of May. This shows that when all actors, especially individual citizens, follow good public health practices, the risk of infection diminishes considerably,” she said in a press release on Tuesday.

When it came to the areas with the highest prevalence of antibodies in blood samples, Ontario had the highest rates at 0.96 per cent and Prince Edward Island had the lowest at zero per cent.

Although Quebec wasn’t included in the study, another recent seroprevalence study by Hema-Quebec found that 2.23 per cent of Quebec blood donors were infected with the virus that causes COVID-19.

“When we take a look at selected cities across the country, Ottawa shows the highest percentage of people with SARS-CoV-2 antibodies with 1.29 per cent seroprevalence, Toronto was found to have 1.07 per cent, whereas Edmonton has the lowest percentage of positive tests at 0.38 per cent,” Timothy Evans, the executive director of CITF, said in a release.

While low rates of seroprevalence suggests there has been low exposure in the overall population, the researchers warned that also means the country is far off from achieving any kind of “herd immunity” and continued vigilance to protect against the virus is still very much needed, particularly now that some parts of the country are experiencing surges of new cases.

“Global data suggest that population-wide infection rates estimated from SARS-CoV-2 antibodies are about 50 per cent higher than those measured in blood donor samples. But 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 David Naylor said.

“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.” 

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