Reinfection reports are still rare but steadily growing around the world, and they’re likely underreported.
There have been confirmed cases of reinfection in Hong Kong, Belgium, the Netherlands, and Ecuador. The Centers for Disease Control and Prevention (CDC) states that “cases of reinfection with COVID-19 have been reported, but remain rare” in the United States.
Reinfection is common with other coronaviruses, like the ones that cause the common cold, but “they often occur after a year or so,” of being ill, says infectious disease expert Amesh A. Adalja, M.D., senior scholar at the Johns Hopkins Center for Health Security. “They don’t usually cause severe disease.” He emphasizes that COVID-19 reinfection “has only happened a handful of times.”
To count as a case of reinfection, a patient must have had a positive polymerase chain reaction (PCR) test twice with at least one symptom-free month in between. But virologist Chantal Reusken of the Dutch National Institute for Public Health and the Environment (RIVM) explains that a second test can also be positive because the patient has a residue of nonreplicating viral RNA from their original infection in their respiratory tract, because of an infection with two viruses at the same time or because they had suppressed but never fully cleared the virus.
Differentiating between the first and second infection
To demonstrate reinfection, scientists have to isolate the microbial culprit each time, check its genetic fingerprint, and show that each infection was caused by a different strain of SARS-CoV-2.
In 10 of the 19 cases tracked where full details are known, the second infection produced worse symptoms than the first, and in five instances it produced serious illness. Researchers think one reason could be that on the second occasion the patient was exposed to a higher or more virulent dose of the virus. It’s unknown whether those who are reinfected can transmit the virus to others.
In light of the confirmed reinfections, a group of researchers recommended in the Lancet that those who’ve had Covid-19 should take the same precautions as everyone else against it.
Some scientists worry about another scenario that could make the second episode worse: enhanced disease, in which a misfiring immune response to the first infection exacerbates the second one. In dengue fever, for example, antibodies to an initial infection can actually help dengue viruses of another serotype enter cells, leading to a more severe and sometimes fatal second infection. In some other diseases, the first infection triggers ineffective, non-neutralizing antibodies and T cells, hampering a more effective response the second time around.
Many experts believe that cases in which patients test positive for the second time could also be due to testing errors, explains Richard Watkins, M.D., infectious disease physician, and professor of internal medicine at the Northeast Ohio Medical University. This could include false positives and negatives due to contaminated samples, human errors, or overly sensitive tests.
How long does COVID-19 immunity last?
Once infected with the coronavirus, nearly everyone with a normally-functioning immune system will develop IgM and IgG antibodies, which can take days to weeks to develop in the body after you have been exposed. While the FDA says it’s “unclear” how long COVID-19 immunity lasts, a new study has some hopeful insight.
The study, which has not yet been peer-reviewed or published in a scientific journal, analyzed antibodies in 185 COVID-19 cases. “We found that the body’s immune system remembers the novel coronavirus from six to eight months, which suggests to us that the immune system can remember the virus for years,” says study co-author Shane Crotty, Ph.D., a professor in the Center for Infectious Disease and Vaccine Research at La Jolla Institute for Immunology. “Most likely, many people will be protected from a repeat case of COVID-19 for a substantial time.”
The study also found some variation in immunity among the participants. “Most people had a measurable robust immune memory but a few individuals did not,” says study co-author Alessandro Sette, Dr.Biol.Sci., a professor in the Center for Infectious Disease and Vaccine Research at La Jolla Institute for Immunology. “We don’t know for sure yet but it is possible that at least some with particularly low immune memory could be susceptible to reinfection.”
It’s unclear, though, why a select few people may wind up with COVID-19 twice. “We don’t know if they just didn’t develop a good immune response beforehand or if they have some sort of genetic nuance that makes them more susceptible,” says Thomas Russo, M.D.,
professor of medicine and chief of infectious disease at the University at Buffalo in New York.
Identifying reinfections is tricky
Not only does it take a while for subsequent bouts to show up, but health departments must also make sure that alleged cases really are reinfections because coronavirus residue can linger for weeks.
Immunity may wane over time—just like it does with other kinds of coronaviruses—and getting sick may even prime some people to suffer worse symptoms if they catch the virus a second time.
It’s possible to get COVID-19 twice, but it seems to be very rare.
It’s important to remember that COVID-19 is caused by a newly discovered coronavirus and there’s still a lot that experts don’t know about it. That includes COVID-19 symptoms, side effects, treatments, and how it responds to a vaccine.
That’s why you should try to avoid being infected in the first place—or being reinfected if you’ve already been sick, as researchers are still studying how the virus behaves and evolves.