Blood donor study suggests more than 120,000 Quebecers have been infected with COVID-19

A study of recent blood donors suggests three times as many Quebecers could have been infected with the novel coronavirus than is reflected in the official numbers.

Highest share of donors who at some point had the virus live in Montreal, Laval

The study analyzed blood samples from 7,691 Quebecers to check for COVID-19 antibodies. (Ryan Remiorz/The Canadian Press)

A study of recent blood donors suggests three times as many Quebecers could have been infected with the novel coronavirus than is reflected in the official numbers.

The study, conducted by Héma-Québec in collaboration with the Institut national de santé publique, confirmed that 2.23 per cent of 7,691 people who donated blood between May 25 and July 9 had contracted COVID-19.

Based on the sample data, the study extrapolates that 124,880 Quebecers aged 18 to 69 had been infected between the start of the pandemic and early July. In the same time period, the study notes that Quebec's Health Ministry had recorded fewer than 40,000 Quebecers ages 20 to 69 as confirmed cases.

As of Tuesday, there are 59,845 confirmed cases, of all ages, in the province.

Blood samples from those living in Montreal and Laval, where the majority of Quebec's confirmed cases have been detected, were most likely to show signs of COVID-19, at 3.05 per cent.

But proportionally, nearly as many donors from the Mauricie-Centre-du-Québec region were also determined to have been infected, roughly triple the region's infection rate. 

The study was conducted by analyzing blood samples for COVID-19 antibodies, which help the body fight off infection. This is different from COVID-19 screening tests, which determine whether someone is currently infected.

It is one of several studies being conducted across Canada to help determine the extent of infection in the country and measure potential immunity to the virus.

In Ontario, a study analyzing blood samples collected in June suggested that 1.1 per cent of the province's population had been infected.

A national study of blood samples conducted by Canadian Blood Services and Canada's COVID-19 Immunity Task Force found that less than one per cent of samples had COVID-19 antibodies.

Results predictable, say experts

Dr. Matthew Cheng, a microbiologist and infectious disease specialist at the McGill University Health Centre and a member of Canada's COVID-19 Immunity Task Force, says he's not surprised the study shows a higher infection rate than previously reported. 

"Not everyone who has the infection is symptomatic, not everyone who is symptomatic is tested and we know that the test has a margin of error," he said in an interview with CBC News.

Getting a better sense of just how many Quebecers contracted COVID-19

2 years ago
Duration 1:47
New study of recent blood donors suggests actual number of Quebecers infected with COVID-19 is three to four times higher than official tally. CBC's Jaela Bernstien reports.

"These results are pretty much where we expected them to be," he said. 

Antibodies are a "good surrogate measure" to see who was infected, according to Cheng.

But it is still unclear if those antibodies will prevent anyone from contracting COVID-19 again, according to Health Canada.

Marc Germain, the vice president of medical affairs at Héma-Québec, said even if the antibodies do provide some level of immunity, the study proves this is not a time for Quebecers to lower their guard.

"We are far from having reached what is called herd immunity," Germain said, noting that most Quebecers are still susceptible to infection.

Germain said additional tests should be conducted to better prepare for future waves of the pandemic. 

With files from Jaela Bernstien and Jennifer Yoon

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