A large majority of Canadians now bear the signs of COVID-19 in their blood in two ways: through vaccination and through infection, a study of antibody levels across the country has found. The findings dictate Canada’s progression to hybrid immunity.
“The most important take home message is that the vaccine compliance of Canadians was amazing,” said Bruce Mazner, a senior scientist at the McGill University Health Centre in Montreal and associate scientific director for strategy with the task force.That compliance, he said, helped to keep the infection rate relatively low, even after vaccines were introduced in 2021 and public-heath measures began to ease.
All of the estimates are based on antibodies that are generated by the immune system in response to proteins from SARS-CoV-2, the virus that causes COVID-19, and spotted in donated blood samples. Among them are antibodies that attach to the spike protein of the virus. Because all the of the vaccines that were approved in Canada target the spike, the presence of those antibodies could be caused by exposure to through infection or through vaccination.
During the first phase, infections were quite low, with only 0.3 per cent of the population exposed to the virus by July of 2020. A year later, once governments had enough doses to distribute vaccines widely, spike protein antibodies climbed steeply in blood samples. “We probably need to think about our interventions and responses that account for that variation because it’s not one size fits all,” said David Buckeridge, a professor at McGill’s School of Population and Global Health who leads data management and analysis for the task force.