Color & Control:

Recovery and recurrence: Untangling the risk of COVID-19 reinfection

By Dr. Caroline Quach-Thanh

Since the beginning of 2020, scientists everywhere have been working around the clock to learn as much as possible about SARS-CoV-2 (the virus behind the COVID-19 pandemic). Tremendous progress has been made already, but post-infection immunity is still something of a puzzle.

“Are you protected after you have it? That’s one of our biggest questions right now,” explains Dr. Caroline Quach-Thanh, a medical microbiologist and pediatrician at the Centre hospitalieruniversitaire (CHU) Sainte-Justine and new Canada Research Chair in Infection Prevention and Control. “With other coronaviruses, we know that people can get reinfected after 3 to 6 months, usually with less severe symptoms—but they still ‘shed’ virus particles, meaning they could still transmit the virus to others. Is that going to be the case with SARS-CoV-2? We just don’t know yet.”

Given the pressing nature of the question and the worldwide focus on the pandemic, Dr. Quach-Thanh expected to see studies and evidence taking shape, but the data remained sparse over the course of the spring and summer.

“If you want the answer, sometimes you have to do it yourself,” she says, which is how she came to lead a Montreal-based team investigating the risk of reinfection. “And we may still see studies from other countries in the coming months, but our health care systems will be different and our patient outcomes and disease severity may be different. No matter what, this warrants a study in Canada.”

Dr. Quach-Thanh’s team will study health care workers who have already been diagnosed with COVID-19 and recovered. Focusing on the Montreal area, one of the hardest hit regions in Canada, the team will recruit 735 hospital-based and long-term care facility staff, including housekeeping and security personnel. These volunteers will undergo blood tests every three months to allow the team to measure their antibody levels and their cell-mediated immune response (the body’s line of defense against pathogens that uses specialized cells instead of antibodies), noting any changes over the course of one year. The volunteers will also complete surveys every two weeks to document any symptoms of illness—and anyone who does show symptoms will be tested again for COVID-19. For anyone who tests positive, the team will dig further to determine if there are any differences between the new infection and the original SARS-CoV-2 infection (such as a different strain of the virus).

Recruitment for the study began in August, but it is hard to predict when the team will have enough data to answer that big question about reinfection. “The timing all depends on how the next wave hits,” Dr. Quach-Thanh explains. “Most of our volunteers are at the 3- to 5-month mark, in terms of how long it has been since their initial infection, so they may be protected—or, if this is like other human coronaviruses, then maybe this is when we’ll start to see that protection fade.”

Volunteers will wear their regular personal protective equipment (PPE) at work and follow all safety protocols, but high case numbers and hospitalizations could still lead to exposure. If, however, the infection rates in the Montreal area can be controlled, then lower case numbers would lead to a lower risk of re-exposure and reinfection for health care workers—a scenario that would still be good news for Dr. Quach-Thanh.

“We are following the dynamics of the immune response in people, which could lead to an interesting result on its own, whether the volunteers end up being at risk for reinfection or not,” she says, adding that she is part of the COVID-19 Immunity Task Force, a federally funded collaboration of experts working to understand and map the scope of coronavirus infection in Canada. “The Task Force makes it easy for us to share data, so our study will contribute to the broader understanding of SARS-CoV-2 immunology no matter what.”

In addition to this knowledge exchange within the Task Force, Dr. Quach-Thanh’s team is well positioned to share information with federal and provincial policy makers and public health officials. The team will create policy briefs to present new insights and data in an easily digestible format, which means the study has the potential to strengthen public health responses and guidance for everything from worker safety to long-term pandemic management planning.

Dr. Quach-Thanh also believes that the answers from this study could change individual behaviour. “If people think they should try to catch it on purpose or that they can be blasé about precautions because they assume they’ll be immune if they recover, then our study might provide some evidence for rethinking that strategy,” she explains. “If people can be reinfected after only a few months, then those individuals would essentially be taking risks for nothing.”

This article was reposted with permission from Canadian Institutes of Health Research. Visit for more research on COVID-19.

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