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Feb. 25, 2021, 11:38 a.m. EST

Only 50 people are known to have contracted COVID-19 more than once — but new strains have medical experts on high alert

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Jaimy Lee

It’s very rare for someone to get infected with the coronavirus causing the disease COVID-19 a second time, but the few confirmed cases of reinfection tell us that immunity to the virus isn’t guaranteed and that vaccination could become a regular part of our medical care going forward.

There are about 60 confirmed cases of reinfection globally. That’s far less than 1% of the 112 million coronavirus cases reported around the world. BNO News, a Dutch news site, is tracking reinfections at a global level; it’s identified 57 confirmed cases of reinfection along with roughly 11,700 suspected or probable reinfections.

Reinfection is thought to occur when protective antibodies wane in people who have previously contracted the virus or in people who contracted the virus without developing antibodies. 

For some infectious-disease doctors, this isn’t a surprise. 

“I think everyone anticipated that there would be reinfections at some distant point after the initial infection,” said Dr. Bruce Polsky, an infectious-disease physician at NYU Langone Hospital on Long Island. “I don’t think anyone had the expectation that if you had COVID and got over it that you could never get it again.”

The Centers for Disease Control and Prevention defines reinfection as a positive COVID-19 test that occurs at least 90 days after an initial positive test. (This is to differentiate from people with persistent “long COVID” symptoms.) The public health agency says reinfection is rare; however, it also says it expects the number of reinfections to rise as the pandemic continues.

“The probability of SARS-CoV-2 reinfection is expected to increase with time after recovery from initial infection due to waning immunity and possibly genetic drift,” the CDC said in October. 

As the pandemic moves into its second year, this could put some of the roughly 28 million people in the U.S. who have already had COVID-19 at risk of getting it again.

The length of the pandemic coupled with the emergence of the new, more infectious B.1.1.7 and B.1.351 variants may also put people at risk for a second infection with the SARS-CoV-2 virus. The CDC has said the B.1.1.7 strain, which has been confirmed in 1,881 Americans, as of Feb. 23 , may become the most dominant form of the virus in the U.S. by the end of March. The B.1.351 variant, first identified in South Africa, has been detected in 46 people in the U.S.

“If [B.1.351] becomes dominant, the experience of our colleagues in South Africa indicate that even if you’ve been infected with the original virus that there is a very high rate of reinfection to the point where previous infection does not seem to protect you against reinfection,” Dr. Anthony Fauci, medical adviser to President Joe Biden and longtime director of the National Institute of Allergy and Infectious Diseases, told CNN in February. 

What medical research is telling us about immunity right now

Studies about reinfection have had mixed findings so far. (A number of projects studying reinfection have recently been announced, including one at the Indiana University School of Public Health in Bloomington evaluating COVID-19 reinfection in U.S. hotspots and an observational study that plans to assess all reported reinfections in France.)

One recent preliminary study that examined Marine recruits arriving at a base on Parris Island in South Carolina found previous infection with SARS-CoV-2 protected them against reinfection — but only to certain degree. Marines with lower IgG antibody titers and neutralizing antibodies were at higher risk of reinfection, the researchers concluded.

Other research underscores the rarity of reinfection. Researchers in Qatar found that less than 1% of the roughly 44,000 people who had recovered from one case of COVID-19 contracted SARS-CoV-2 a second time.  

But beyond reinfection risk, there are a number of factors that can influence immunity to the SARS-CoV-2 virus. 

Not all people who contract the virus develop antibodies to it, and some people end up with antibody levels that aren’t high enough to protect them.

It’s also unclear how long antibodies stick around. COVID-19 antibodies decline after 60 days in most people, according to a CDC study of frontline healthcare workers in 12 states and another study of the same group in Nashville, Tenn., though other studies including one examining healthcare workers in the United Kingdom have indicated that IgG antibodies may last for six months or longer. 

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