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home / 世界杯决赛2022

covid-19 reinfections are rising — here's why

experts say reinfections don’t mean the vaccines — and our immune systems — aren’t doing their jobs.

by katelyn thomas

apr 28 2022
read time 6 minute read
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a growing number of covid-19 reinfections have been reported in quebec during the fifth and sixth waves, leading many to wonder — what is going on with our immunity?

in some cases, people have been reinfected within mere weeks of a first infection.
earlier in the pandemic, reinfections were rare — accounting for fewer than one per cent of cases in the province. the arrival of the ba.1 omicron variant, which fuelled quebec’s fifth wave in december and january, pushed that to four or five per cent of all infections, according to dr. gaston de serres, an epidemiologist at the institut national de santé publique du québec.
but experts say that doesn’t mean the vaccines — and our immune systems — aren’t doing their jobs.
“the problem is that the virus is continuing to evolve all the time … so it’s very possible to have an infection and a month later, get covid again with a variant that is different enough from what you saw a month ago that it actually evades immunity,” said dr. catherine hankins, co-chair of canada’s covid-19 immunity task force and professor of medicine in mcgill university’s department of epidemiology, biostatistics and occupational health. “the changes in the virus, they reduce antibody recognition, that’s what the virus is trying to do — it’s trying to evade the immune system.”
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dr. andrés finzi, a long-time hiv researcher at université de montréal who holds the canada research chair on retroviral entry, pointed out that it takes time for immunity to build — which could explain reinfections that are unusually close together.
“people got a breakthrough infection and then two weeks later got infected again, it is possible,” he said. “i know people who got vaccinated and infected in the same week, they say ‘the vaccine is not working.’ … unfortunately, there wasn’t enough time for the immune system to build up into a response.”
the immune system needs time to respond in the same way after an infection, he said.
“imagine, if you didn’t need to build immunity, not a single virus would infect us. we would be protected right away.”
it’s also possible that the initial infection was so mild it didn’t elicit a robust immune response, according to immunologist dr. andré veillette from the montreal clinical research institute.
“i think when it’s just like a stuffy nose, or no symptoms, that’s because the infection is controlled locally in the nose, so it’s not going to have an effect all over the body on the immune system” to boost it in the same way a vaccine dose would, he said.

a recent study by the inspq shows that to be true of milder infections. published last week, the study evaluated how people who had covid-19 during past waves fared against the omicron variant. it showed the risk of reinfection was reduced much more in people who had harsher symptoms.

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for example, for quebecers who had covid-19 and were boosted with a third vaccine dose, the risk of reinfection was reduced by 87 per cent among those who were hospitalized, compared with 82 per cent among those who had symptoms and 66 per cent among those who had none.
the discrepancy depending on symptoms exists regardless of vaccine status, but those with more doses were more protected against reinfection. unvaccinated people were the least protected, at 68 per cent among the hospitalized, 43 per cent among those who had symptoms and just eight per cent among the asymptomatic.
how long protection from reinfection lasts also varies depending on vaccine status. while it hovers around 65 per cent for anyone who has been infected for the first two to five months post-infection, it drops to 35 per cent among the unvaccinated nine to 11 months later, but remains at around 62 per cent among those with one or two doses.
“when we’ve been infected and gotten vaccination, protection increases with each dose of vaccine — we looked at first, second, and third doses,” de serres said. “the protection against severe disease seems to stay strong enough even against omicron, and that’s certainly good news.”

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the study suggests that hybrid immunity — gained from infection and vaccination — protects better against reinfection and severe disease than immunity from a single source. still, all forms of immunity are shown to protect better against hospitalization than reinfection, which has been proven during the omicron waves.
“what we show (in the study) is that when we have been infected in the past, the infection of the past provides very good protection against serious disease … a protection that’s less good against light reinfection,” de serres said. “and that the protection against light reinfection decreases with time.”
it’s too soon to say how long protection developed from an omicron infection will last, but de serres said the inspq expects it to fare better against the ba.2 sub-variant compared with how protection from previous variants fared against omicron. so far, data from other countries suggests being infected with omicron and then the ba.2 sub-variant can happen, but that “those events are fairly rare” because they are more similar, de serres said.
“even if there are cases that are reinfected within a month, the large majority of people who’ve had a first infection won’t be reinfected for months,” he said.

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all experts said that given the ultimate goal is to prevent severe disease and death, the vaccines are indeed doing their jobs — even amid high transmission rates and higher cases of reinfection.

finzi said that while antibodies from covid-19 vaccines or earlier variants are less capable of neutralizing omicron and stopping infection, they retained the ability to call the immune system for help.

“so far, we are lucky,” he said of the omicron waves. “protection against severe outcome was there.”
looking to the future, de serres said that when it comes to other respiratory viruses, a first infection has generally meant ensuing ones will be less severe. that was the case during the 2009 outbreak of h1n1, he said, which hit seniors less hard than expected because they’d encountered it earlier in the form of the spanish flu.
“those people had been infected with h1n1 as a first exposure to the flu, and even 60 years later, when a new h1n1 arrived, well they weren’t as affected,” he said.
“to encounter a virus for the first time creates an immune memory that is important and that can last a long time,” de serres said. “and so in that regard, it’s possible that immunity people have might not be sufficient to protect them against developing light symptoms, but it will still be sufficient to prevent the disease from becoming severe, and i think that’s the most important thing.”

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in addition to uncertainty surrounding long covid, some have raised concerns about the potential effects of reinfections — but hankins said it’s much too early to say a reinfection could cause more harm than an initial one.
“it’s a little premature in terms of what we’re seeing,” she said, “because we didn’t really start seeing all these reinfections — they were individual case studies that were so rare — we didn’t see them until omicron came.”
“we don’t know,” veillette added. “i think still the bottom line is it’s a virus you don’t want to catch.”
kthomas@postmedia.com
twitter.com/katelynthomas

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katelyn thomas, montreal gazette
katelyn thomas, montreal gazette

i have been reporting on a range of news at the montreal gazette since 2021, with a particular interest in social issues and human interest stories.

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