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Here’s what a blood study tells us about COVID-19 in Canada

TORONTO — The latest attempt to draw conclusions about the spread of COVID-19 in Canada by analyzing blood donations suggests that racialized populations, young adults and residents of lower-income neighbourhoods are among the hardest-hit by the disease.

The federal government’s COVID-19 Immunity Task Force (CITF) has been monitoring the presence of COVID-19 antibodies in blood donated to Canadian Blood Services and Héma-Québec since early in the pandemic, in an attempt to find clues as to how far the novel coronavirus has spread across the country.

Their latest findings, released Thursday, show that “the pandemic is affecting racialized Canadians twice as much as self-declared Caucasian Canadians,” the task force said in a press release.

In the nine provinces studied by Canadian Blood Services – every province except Quebec – 3.37 per cent of blood samples from donors who did not identify as Caucasian tested positive for COVID-19 antibodies, versus 1.66 per cent of samples from donors who did identify as Caucasian.

“Consistently, from the beginning of the pandemic, seroprevalence rates were higher among racialized populations compared to donors who self-identified as Caucasian. By January 2021, the seroprevalence was about double in racialized donors,” Dr. Sheila O’Brien of Canadian Blood Services, the study’s primary investigator, said in the press release.

Seroprevalence refers to the percentage of tested blood samples in which COVID-19 antibodies were detected.


The researchers reported observing trends in other demographic groups as well, including a continuing slide toward higher infection rates in less wealthy neighbourhoods. While CITF found that seroprevalence did not change much between neighbourhoods of different income levels last spring, during the first wave of the pandemic, that is no longer the case. In January, the task force reports, blood donors living in more affluent neighbourhoods had a seroprevalence rate of 1.2 per cent, versus four per cent in less well-off areas.

The CITF research also provides more evidence that the latest wave of COVID-19 has been affecting younger adults much more than the first two waves did.

During the first months of the pandemic, the seroprevalence rate outside Quebec for blood donors between the ages of 17 and 24 stood at 0.8 per cent. In January, it was 3.5 per cent – more than four times higher. In Quebec, the January seroprevalence rate for those aged 18 to 24 was double that of donors in their 60s.

“The disproportionate number of infections in young people is likely a product of both their front-line work environments and more at-risk behaviours,” CITF executive director Dr. Tim Evans said in the press release.

“This also reflects effective public health efforts to protect seniors in the second wave. The immediate priority is to encourage this younger age group to get vaccinated now that they are eligible in most provinces.”


In all provinces except Quebec, the blood samples that were analyzed for this research were donated in January. The Quebec samples encompassed a wider and more recent timeframe, spanning from January to March.

As a result, the overall seroprevalence rate in Quebec (10.5 per cent) was much higher than that of the rest of the country (2.2 per cent). That difference, CITF says, shows signs of success for Canada’s vaccine rollout. According to CTV News data, the number of vaccine doses administered in Canada at the end of March was nearly six times what it was at the end of January.

In other words, Quebec’s higher seroprevalence rate was not indicative of more COVID-19 infections, but of antibodies being found in blood from donors who had been inoculated against COVID-19. Among donors who reported receiving a dose of a COVID-19 vaccination at least two weeks before they gave blood, the seroprevalence rate was 95 per cent.

Of the nine provinces where only blood samples from January were analyzed, Manitoba had the highest seroprevalence rate at 5.4 per cent, followed by Alberta at four per cent and Saskatchewan at 3.5 per cent.

The lowest seroprevalence rates in the country were found in Atlantic Canada, where virus activity in January was the lowest in the country. Nova Scotia’s seroprevalence rate was below 0.4 per cent, New Brunswick’s below 0.2 per cent, and not a single sample of blood from Prince Edward Island or Newfoundland and Labrador tested positive for COVID-19 antibodies.


Evans said that the overall low rates of COVID-19 antibodies in blood samples “show Canadians have been doing a great job following public health guidelines” but also that “high vaccine uptake” is necessary for the pandemic to end. It is not yet known how long immunity from COVID-19 infection or vaccination lasts.

“Continuing these serosurveys of blood donors will signal when people in different age groups start to show less immunity, which may help to inform the best timing of the possible need for a booster vaccination,” he said.

In fact, CITF reported, there are already signs out of Quebec that some antibodies may be starting to disappear. Dr. Marc Germain, the study’s lead researcher and vice-president of medical affairs and innovation at Héma-Québec, said in the press release that of 109 blood samples, nearly 30 per cent of samples that tested positive for COVID-19 antibodies during the first wave subsequently tested negative seven to 10 months later, and 14 per cent of those who had more severe cases of COVID-19 “lost their antibodies within a year.”

The CITF researchers caution that studies based on blood samples tend to underrepresent young and elderly demographics while overrepresenting the healthiest Canadians and those who have the most time and ability to visit a blood-donation clinic.

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