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

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 27 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 1,523 Americans, as of Feb. 16, 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 21 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 earlier this month. 

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. 

“Because you don’t know how long immunity will be induced, we have the possibility as the virus continues to mutate that it actually mutates to become a cold virus,” said Dr. Stanley Perlman, professor of microbiology and immunology at the University of Iowa. “That’s what we really hope.”

If this were to play out, the virus could be dulled until it simply becomes an annoying but no longer deadly common cold. Common colds, which are coronaviruses that haven’t been eradicated, can provide immunity for up to three years. People who were infected with SARS, or severe acute respiratory syndrome, also a coronavirus, had about three years of immunity.  

“Sterilizing immunity” is a term used to explain a type of immunity that means people cannot contract either a virus or the disease caused by a virus. That’s most likely not going to happen with SARS-CoV-2. What experts including Perlman are now wondering is if reinfections will cause less severe forms of the disease or be asymptomatic.

“That could well be what’s happening with people who have mild disease and get reinfected with severe disease,” Perlman said. “I think the protection is much longer. So if somebody comes out of an ICU and is otherwise normal, I think that they will be protected for years.”

What we know about reinfection rates in the U.S.

The federal government does not track reinfections, and so the number of reinfection cases in the U.S. is unknown. The page on the CDC website with information about reinfections was last updated Oct. 27. 

Some states have shared details about probable or confirmed reinfections upon request. 

California, for example, lists two confirmed cases of reinfection out of the 3.3 million people who have tested positive for the virus there, according to the state’s public health department. About 615 people in Colorado meet the CDC criteria for reinfection, according to a state spokesperson. “This is a very small percentage of all cases,” making up an estimated 0.15% of total cases in Colorado, she said in an email. In Washington state, there are 716 suspected reinfections, and one confirmed case.

The first documented case of reinfection in the U.S. occurred in a 25-year-old man in Reno, Nev., according to a study published in the Lancet medical journal in October. The individual tested positive in April, had two negative tests in May, and then tested positive again in June. 

His case was identified as part of a genomic sequencing program that had been set up in Nevada during the pandemic. (The only way to confirm a reinfection is to sequence both samples to ensure they have different mutations, which naturally occur as the virus spreads from person to person.)

“If we look at differences from the first sample compared to the original [virus from Wuhan, China] that are absent from the second … that sort of proves that they were different, that they had diverged, before they could have entered that person,” said Richard Tillett, a biostatistician for the Nevada Institute of Personalized Medicine and a co-author of the Lancet study. 

What to know about reinfection and vaccines

If SARS-CoV-2 isn’t going away, reinfection could become more likely, even if the virus changes shape to become less deadly or to cause less severe illness.

Vaccine developers like Johnson & Johnson JNJ, -1.67% and Moderna Inc. MRNA, +3.05% have said they are considering programs to develop COVID-19 boosters or vaccines that are administered on a regular basis into the future. 

“For the next several years, we’ll be getting a COVID-19 shot just like we get a flu shot,” J&J CEO Alex Gorsky told CNBC on Feb. 9. “We can all imagine a future where we’re living with this, but we can keep the science at pace with the virus.”

As of now, the CDC says that people who have been infected once with the virus should still get vaccinated, as long as they wait at least two months after a negative test. 

The late-stage clinical trials for both authorized vaccines did not study, for the most part, the impact of the vaccines on people who had previously been infected with the virus. But Pfizer Inc. PFE, -0.35%, with German partner BioNTech BNTX, +2.70%,  and Moderna separately told the Food and Drug Administration during the regulatory review process that there are “limited” data suggesting people who have had the virus can be at risk for reinfection and could benefit from vaccination.

“We’ve seen some case reports here and there of people getting reinfected,” Dr. Tal Zaks, Moderna’s chief medical officer, said in January during an investor call. “But in a context of millions getting infected, I don’t think that is material.”

Some healthcare workers at Hackensack Meridian Health hospital system in New Jersey asked administrators if they should get vaccinated if they’ve already been infected. The short answer is yes, according to Dr. Daniel Varga, Hackensack’s chief physician executive. 

“We’re putting those people in the same PPE,” he said, “because you don’t know exactly how long and how effective their acquired immunity is.”