For the nearly 100 million people around the world who’ve been infected with the coronavirus, new science offers some comfort: Reinfections appear to be rare, and you may be protected from Covid-19 for at least five months.
The study, the largest of its kind, followed more than 20,000 health workers in the UK, regularly testing them for infection and antibodies. Between June and November, the researchers — from Public Health England (PHE) — found 44 potential reinfections out of the 6,614 participants who had tested positive for antibodies or had a previous positive PCR or antibody test when they joined the study. Meanwhile, of the 14,000-plus people who had tested negative for the virus at the start of the study, there were 409 new infections.
Only two of the 44 potential reinfections were designated “probable” and the rest were considered “possible,” “based on the amount of confirmatory evidence available,” according to the health agency. Fifteen people — or 34 percent — had symptoms.
So if all 44 reinfections are real, that translates to an 83 percent lower risk of reinfection compared to health workers who never had the virus. If only two are confirmed, that rate of protection goes up to 99 percent. Either way, it suggests natural immunity might provide a similar level of protection as the approved Covid-19 vaccines.
But as with the vaccines, it’s not yet clear how long immunity after an infection lasts. Antibodies may fade after five months or last much longer, something the researchers behind the ongoing study, which will run for a total of 12 months, plan to investigate.
“This [new] study does provide some comfort that naturally acquired antibodies are pretty effective in preventing reinfections,” Akiko Iwasaki, an immunobiologist at Yale University, told Vox. The findings also square with another paper on health workers, published in the New England Journal of Medicine in December: Researchers found people who had Covid-19 antibodies were better protected from the virus for six months than people who did not.
Iwasaki added, “You can also interpret these data to mean that protection against reinfection is not complete — especially for people who had Covid during the first wave, say in March-April 2020.”
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People who had the virus may still be able to pass it on if reinfected
The good news for individuals who have had Covid-19 also comes with a warning about the risk they can still pose to other people. While antibodies might protect against a second case of Covid-19 in most people, “early evidence from the next stage of the study suggests that some of these individuals carry high levels of virus and could continue to transmit the virus to others,” PHE warned in its press release.
“We now know that most of those who have had the virus, and developed antibodies, are protected from reinfection, but this is not total,” Susan Hopkins, a senior medical adviser at PHE and the study lead, said in a statement, “and we do not yet know how long protection lasts.”
In other words, even if you’ve had Covid-19, while you’re unlikely to get really sick again anytime soon, you should still consider yourself a potential risk of spreading it to others if you catch the virus again and are asymptomatic. That means continuing to take precautions — like mask-wearing and social distancing, Iwasaki added. And it’s one reason why immunologists have said people who’ve already been infected with the virus should still plan to get the vaccine when their turn comes.
So there’s still a lot more to learn about immunity after Covid-19: How will the new coronavirus variants affect it? Lab data from South Africa, where the 501Y.V2 variant has been spreading, suggests it might be able to escape antibodies produced by prior infections in some people.
Who is most likely to have a strong immune response? We do have some evidence that different individuals mount different antibody responses after Covid-19 infections, but the PHE researchers found no statistically significant difference in rates of protection between people who reported symptoms and those who did not. It’s also possible factors like gender and disease severity influence the strength of a person’s immune response.
For now, though, the research suggests that survivors of the virus might just help us get to herd immunity faster — if their immunity lasts long enough. But given the virus has only been known to humans for a little over a year, it may take a while to authoritatively answer the question.