Scientists explore a simple question with big implications: Can people be reinfected by COVID-19?

As scientists try to anticipate how the COVID-19 pandemic will play out in the second half of this year and beyond, one key unknown factoring into their calculations is the question of how long immunity to the virus lasts.

The answer can be determined only with time. So far, limited reports of reinfection by the virus from South Korea and China are inconclusive and could be due to tests picking up the residue of an initial infection that has not yet cleared.

But in Canada experts stress that organized monitoring for changing immunity levels to COVID-19 should begin now to deliver meaningful data down the road.

“You have to perform population-based studies with enough numbers to actually make conclusions rather than guesses,” said Charu Kaushic, an immunologist at McMaster University and scientific director of the Institute for Infection and Immunity, a federal granting body.

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Dr. Kaushic, who is a member of Canada’s newly formed immunity task force, said that this week the 20-member group began discussing precisely how such studies should be conducted.

The implications are enormous, particularly before a vaccine is developed and made widely available. A Harvard University analysis published last month found that if people can be reinfected with COVID-19 within a year or two, the virus will likely take up permanent residence in the population. Only if immunity endures beyond a few years is there a chance that the virus will die out after the initial pandemic wave has run its course.

Which way things go will depend, in part, on proteins called neutralizing antibodies, which the immune system usually produces once it has experienced a particular virus to help fight off a subsequent infection. Typically, such antibodies indicate a degree of protection, although how long the protection lasts may depend on both the virus and the severity of the initial infection.

Other human coronaviruses offer mixed signals that make it hard to predict how things will go with COVID-19. On one hand, the four types of human coronaviruses that are known to cause colds tend to show up in infancy and return again and again. Their relatively high reinfection rates guarantee that they are endemic to humans.

On the other hand, data gathered during the SARS outbreak of 2003 suggest that individuals who were infected with that far more serious coronavirus maintained antibodies for at least three years. This suggests a longer duration of immunity, though how long remains unclear since SARS was gone before those who survived it had to face it again.

“The takeaway is that getting sicker gives you a stronger [immune] response,” said Derek Cummings, a specialist in pathogen transmission at the University of Florida in Gainesville who recently conducted a systematic review of previous coronavirus research. “We don’t know if COVID-19 is going to be different.”

On Tuesday, researchers based at Mount Sinai hospital in New York reported that more than 99 per cent of those who had COVID-19, drawn from a study of 1,343 individuals, formed neutralizing antibodies to the virus. The researchers said they plan to follow their study group for the next six months to track antibody levels across time.

While this is an important step toward determining immunity, it’s clear that more widespread data will be needed. For example, the New York study sample was made up of hospital outpatients, many of whom had mild versions of COVID-19. That means the results do not address whether someone who was infected but showed no symptoms formed antibodies to the disease.

One way around this is to leverage long-running studies that have already been formed and that can track immunity indicators across a broad swath of the population.

Susan Kirkland, another immunity task force member and a co-principal investigator of the Canadian Longitudinal Study on Aging, said she is now preparing a proposal to do just that.

The long-running study, which tracks the health of more than 50,000 Canadians over the age of 45, is well set up to collect data, including blood samples, from a large cohort of randomly selected participants.

“We realize we have a really good opportunity to study COVID-19 in our group,” said Dr. Kirkland, who is head of community health and epidemiology at Dalhousie University.

The fact that the group is made up of older Canadians – among the most vulnerable to COVID-19 – and, apart from the north, is geographically representative of most of the country means that it could offer a powerful window for tracking the ebb and flow of immunity across the country.

On a smaller scale, but more directly at the front lines of the infection, a Toronto-based clinical trial of about 1,500 hospital workers called the RESPECT trial offers another avenue for exploring how immunity wanes among those most likely exposed to the virus after having had COVID-19. However, Deepali Kumar, a researcher with University Health Network who is leading the trial, said that improved access to personal protective equipment in hospitals should be making the chance of reinfection less likely.

“The best way to measure protection may be during a [second] outbreak which we all hope won’t happen,” she said.

James Kellner, a professor of pediatrics at the University of Calgary who is also on the task force, said that making good use of blood samples from cohort studies will require identifying antibody tests that are highly reliable, another priority of the task force. Even then, he added, it’s important to know that measuring antibodies alone will not provide the full picture of immunity because of other mechanisms that the body employs to fight viruses.

“Even if your antibodies fall away, you may still be resistant to being reinfected … that’s a subtle but key point,” he said.

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