Covid-19 survivors have a low risk of reinfection

New government-funded research this week should offer some comfort to people who are covid-19 survivors. This suggests that they have a low risk of re-infection with the coronavirus, at least about three months later.

Researchers from the National Cancer Institute partnered with commercial test labs and two health data collection companies for this study, published Wednesday in Internal Medicine JAMA.

They analyzed anonymized data from more than 3 million Americans who had undergone commercial antibody testing for SARS-CoV-2, the coronavirus responsible for covid-19, between January and August 2020. Antibody tests , although not perfect, indicate if anyone has had a recent infection. These people were divided into those who had antibodies and those who did not, according to the tests. Next, the researchers looked at how many people in both groups subsequently had a PCR test for covid-19, which aims to diagnose an active infection.

About 10% of people in each group then underwent 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 this is not surprising, because 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. It is therefore likely that these positive PCR results usually detect 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 true reinfection – the results were encouraging.

After three months or more, only 0.3% of people with a previous positive antibody test tested positive for the coronavirus again, 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 months or more later.

“People who have recovered from covid-19 should be reassured that being positive for antibodies is associated with some protection against further infection,” said study author Douglas Lowy, deputy director principal of NCI, in an email.

The results have their limits, however. On the one hand, they can’t tell us exactly how much protection a previous infection will provide against re-infection, or how long it should last (although other research has suggested that it may take years). Another factor that this study cannot explain is the recent emergence of coronavirus variants. Some – like this one first identified in South Africa last year – they are believed to increase the risk of re-infection, as they may be able to partially evade the immune response created by a previous infection or vaccination.

Yet, no research shows that spreading variants can completely escape the immunity natural or provided by a vaccine. Our immune system has lots of weapons against a familiar germ, and it’s likely that most re-infections will turn out to be milder than the first time.

Even before these new variants existed, however, there had been documented cases of reinfection, including case where symptoms were worse on the second go-around. And the results of the new study still suggest that reinfection does occur, so rarely. So no one should assume that they are impervious to covid-19 just because they have survived a previous infection without a problem. Ultimately, the best way to protect everyone from covid-19 is to vaccinate as many people as possible, including those who have had the viral illness before., according to Lowy. It is a remedy that carries much less risk than getting a natural infection.

“People who have recovered from covid-19 should always plan to be vaccinated when they get the chance,” he said.

The NCI plans to continue funding research that will track the prevalence of re-infection in the general public, as well as studies that will examine how our immune response to the virus may change over time and against new variants.