New government-funded research this week should provide some comfort to people who survived Covid-19. It suggests they have a low risk of coronavirus reinfection at least about three months later.
Researchers from the National Cancer Institute teamed up with commercial testing labs and two healthcare data collection companies for this study, published Wednesday at JAMA Internal Medicine.
They analyzed anonymized data from more than 3 million Americans who had received a commercial antibody test for SARS-CoV-2, the coronavirus that causes covid-19, sometime between January and August 2020. Antibody tests, while not perfect, indicate if someone has a had any previous recent infection. These people were divided into those who had antibodies and those who didn’t, based on testing. Next, the researchers looked at how many people in both groups were later given a PCR test for covid-19, which is designed to diagnose an active infection.
About 10% of the people in each group received a PCR test. More people with antibodies tested positive for the virus within the first 30 days of their antibody test than people without antibodies. But that’s not surprising, since detectable traces of the virus can remain in the body for months, even after the symptoms have passed and the person is no longer contagious. So it is likely that these positive PCR results usually picked up the first infection. When the researchers looked specifically at the number of positive tests 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 more, only 0.3% of those with a previous positive antibody test retested positive for the coronavirus, compared to 3% of those with a negative antibody test. In other words, having a previous infection was linked to a much lower risk of infection three or more months later.
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“People who have recovered from covid-19 should be assured that being antibody positive is associated with some protection against new infection,” said study author Douglas Lowy, the NCI’s chief deputy director, in an email.
However, the findings have their limitations. First, they can’t tell us exactly how much protection a previous infection provides against reinfection, or how long it is expected to last (although other research has suggested that it could be years). Another factor that this study cannot take into account is the recent emergence of coronavirus variants. Some – like that first identified in South Africa last year – are believed to increase the risk of reinfection as they may be able to partially bypass the immune response triggered by a previous infection or vaccination.
Still, there’s no research showing that currently spreading variants can completely bypass a person’s natural or vaccine-provided immunity. Our immune system has lots of weapons against a known germ, and it is likely that most re-infections will turn out to be milder than the first time.
But even before these new variants appeared, there were documented cases of reinfection, including cases where symptoms were worse on the second go-around. And the new study’s findings still suggest that reinfection is occurring, albeit rarely. Thus, no one should assume they are refractory to covid-19 just because they survived a previous infection without problems. Ultimately, the best way to protect everyone from Covid-19 is to vaccinate as many people as possible, including those who have already had the viral disease., says Lowy. It is a remedy that carries much less risk than getting a natural infection.
“People who have recovered from Covid-19 should still plan to get vaccinated if given the opportunity,” he said.
The NCI plans to continue to fund research that will track the prevalence of re-infection in the general public, along with studies that will look at how our immune response to the virus can over time and against new variants change.