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, told CTV News that while this is an indication that Canadians have done well to limit the spread of COVID-19 curve, it also means that we are still a long ways away from achieving any sort of herd immunity and are at risk of a second wave.
"I would say this does mean that the fall is looking daunting,” she said. “We don't have a lot of immunity in the population and we need to continue to be really vigilant: hand washing, physical distancing, mask wearing.”
While several vaccines around the world are being tested, Sheila O'Brien, associate director of Epidemiology and Surveillance at Canadian Blood Service, warns that this data shows Canadians will not be protected from COVID-19 until such a vaccine is ready.
“It certainly does show that we do not have herd immunity and that we are not going to have herd immunity until a vaccine becomes available,” she said.
Hankins said in a news releasethat 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.
Still, even if the estimates are slightly off, CITF Co-Chair David Naylor said the report still shows the need for increased testing and contact tracing.
“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,” he said in the report. “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.”
“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.”
Meanwhile, other studies suggest that COVID-19 antibodies do not tell the whole story and that other cells in the immune system may be better for determining immunity, such as T cells, which can possibly offer immunity through memory of other viruses within the coronavirus family.
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.
With files from CTV News’ medical correspondent Avis Favaro, CTV National News senior producer Elizabeth St. Philip and CTVNews.ca Writer Ben Cousins
