New government-funded research this week should offer some comfort to people who have survived covid-19. It suggests they have a low risk of reinfection from the coronavirus, at least around three months later.
Researchers at the National Cancer Institute teamed up with commercial testing labs and two healthcare data collection companies for this study, published Wednesday in JAMA Internal Medicine.
They analyzed de-identified data from more than 3 million Americans who had gotten a commercial antibody test for SARS-CoV-2, the coronavirus that causes covid-19, sometime between January to August 2020. Antibody tests, while not perfect, indicate whether someone has had a prior recent infection. These people were divided into those who had antibodies and those who didn’t, based on testing. Then the researchers looked at how many people in both groups later got a PCR test for covid-19, which is meant to diagnose an active infection.
About 10% of people in each group went on to get a PCR test. More people with antibodies tested positive for the virus within the first 30 days after their antibody test than those without antibodies. But that’s not surprising, since detectable traces of the virus can remain in the body for months, even after symptoms have passed and the person is no longer infectious. So it’s likely these positive PCR results were usually picking up the first infection. When the researchers looked specifically at the positive test rate after the first month and especially more than 90 days later—enough time for a positive PCR test to likely indicate a true reinfection—the results were encouragingly different.
After three months or longer, only 0.3% of people with an earlier positive antibody test tested positive for the coronavirus again, compared to 3% of those with a negative antibody test. In other words, having a past infection was linked to a much lower risk of infection three or more months later.
“People who have recovered from covid-19 should be reassured that being antibody-positive is associated with some protection against a new infection,” study author Douglas Lowy, the principal deputy director of the NCI, said in an email.
The findings do come with their limitations, though. For one, they can’t tell us exactly how much protection a past infection will provide against reinfection, or how long it’s expected to last (though other research has suggested that it may be years). Another factor this study can’t account for is the recent emergence of coronavirus variants. Some—like the one first identified in South Africa last year—are thought to raise the risk of reinfection, since they may be able to partly evade the immune response created by an earlier infection or vaccination.
Still, there’s no research showing that any currently spreading variants can completely evade someone’s natural or vaccine-provided immunity. Our immune system has plenty of weapons against a familiar germ, and it’s likely that most reinfections will turn out to be milder than the first time.
Even before these new variants were around, though, there had been documented cases of reinfection, including cases where symptoms were worse the second go-around. And the new study’s findings still suggest that reinfection does happen, if rarely. So no one should assume they’re impervious to covid-19 just because they survived an earlier infection with no problem. Ultimately, the best way to keep everyone safe from covid-19 is to vaccinate as many people as possible, including those who have already had the viral illness, according to Lowy. It’s a remedy that involves a lot less risk than getting a natural infection.
“People who have recovered from covid-19 should still plan to be vaccinated when they have the opportunity,” he said.
The NCI plans to continue funding research that will track the prevalence of reinfection in the general public, along with studies that will look at how our immune response to the virus may change over time and against new variants.