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Blood samples are prepared for placement into a centrifuge to separate plasma for antibody testing at the Bloodworks Northwest Laboratory in Renton, Wash., on Sept. 9, 2020, amid the COVID-19 pandemic.LINDSEY WASSON/Reuters

Canada has begun its largest effort so far to determine how many people in the country have had COVID-19.

For the project, led by the federally appointed COVID-19 immunity task force, 48,000 finger-prick blood collection kits will be shipped to individuals across the country over the coming months. Those who receive the kits will be asked to respond to an online questionnaire and provide blood spots to be checked for the presence of antibodies that can indicate whether they have been infected with the coronavirus during the past several months.

The goal is to reveal the extent to which COVID-19 has penetrated different segments of the Canadian population and regions. The data will help track the spread of the coronavirus and allow researchers to better estimate how many people may have had COVID-19 in Canada, including those who did not experience symptoms. Results of the survey may also show how the severity of the disease varies among different groups and help officials determine the best way to distribute vaccines when they become available.

Four thousand kits were mailed out last week as part of an initial round of the survey to gauge public response. Those should provide a preliminary national estimate of how prevalent the disease has been until now. The remaining 44,000 kits will be distributed in the new year in two batches between January and April that will enable the task force to provide estimates of infection rates by province.

Catherine Hankins, a professor of public and population health at McGill University and the co-chair of the task force, said that while the survey is designed to produce meaningful results with as few as 45 per cent of the kits returned, she and her colleagues hope for a far higher response rate.

“So far, participation has been great. People want to know their own results and also want to help," she said.

Dr. Hankins stressed that when a collection kit arrives in someone’s mailbox, it is important that the person tested is the one who is indicated by the survey – or who falls into a specified category, such as the youngest or oldest member of the household – to ensure a nationally representative sample.

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The immunity task force was formed last spring to measure exposure and potential immunity to COVID-19 at the population level. Early on, it worked with Canadian Blood Services, which randomly tested donated blood serum for antibodies that the body forms naturally in response to COVID-19 and that can linger for months. The results, released in July, showed that less than one per cent of Canadians had been infected by June.

While this provided an initial snapshot, a survey of blood donors leaves out many population groups, including children, people who live far from blood donation centres and those who are in poor health. Dr. Hankins said the new survey was created in partnership with Statistics Canada to address those gaps and to pull in data from a much larger sample whose diversity and distribution closely mirrors the population of the country.

The task force is also working with other investigators targeting specific groups to get a more detailed view of infection rates in different population cohorts.

“We’re trying to put different pieces of the puzzle together to get a more comprehensive national picture of what’s happening,” Dr. Hankins said.

Meanwhile, scientists behind the first large-scale survey of COVID-19 prevalence in Canada, which was launched last May, say they will soon be ready to release their findings. That study, conducted by St. Michael’s Hospital in Toronto in conjunction with the polling company Angus Reid, sought to collect blood from 10,000 people with a response rate that is close to 80 per cent.

Prabhat Jha, a physician and director of the hospital’s Centre for Global Health Research, said it took longer than he had hoped to conduct the study at a time when tests for COVID-19 antibodies were still being developed and validated.

Now, all of the blood samples the study received over the summer have been checked for antibodies using a commercial test approved by Health Canada. The samples will be tested again this month with a more sensitive screen to reduce the rate of false positives. Once the second round is completed, study participants will be told whether there is evidence that they had COVID-19 last spring.

“The study will be able to comment on [Canada’s] experience with the first viral wave,” Dr. Jha said.

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