The first case of reinfection with COVID-19 has been confirmed in the U.S. The reinfected patient is a 25-year-old man from Nevada who was diagnosed first in April and then again six weeks later after two negative tests in between. This is the fifth confirmed case of COVID-19 reinfection in the world, although about 20 more are unconfirmed and others have probably gone undetected. And while it’s rare, reinfection has two very important implications: 1) If you’ve been infected and recovered, you still need to wear a mask and social distance, and 2) Herd immunity is not a viable approach to ending the pandemic.
According to Danny Altmann, a professor of immunology at Imperial College London, about 90 percent of people with symptomatic COVID-19 will develop the antibodies they need to defend against the coronavirus for a year or so. That means 10 percent could still get reinfected within that year. “[T]hey have precisely the same risk as anyone out there, thus a small but significant number of reinfections,” he told NPR in an email.
But that’s just one man’s guess. Other experts suspect that reinfection is much rarer. “You’ll never have the distribution of anything with millions of people where you don’t have some very severe rare cases happening at the fringe,” Michael Mina, a pediatric immunologist at the Harvard T.H. Chan School of Public Health, told the New York Times.
Despite all the talk, experts have no way of knowing how common reinfection actually is. Asymptomatic reinfections are almost definitely underreported, so the number could be much higher than what researchers estimate. It’s also difficult to confirm reinfections because to do so experts must test the viruses’ genetic material for differences. Many people with COVID-19 never get tested at all, and only some labs are equipped to analyze viral genomes.
It seems like most people who get reinfected are asymptomatic or don’t get as sick during their second fight with the disease, but some are worse the second time around. That was the case for the Nevada man and another patient in Ecuador. And a Dutch woman died after being reinfected, researchers reported last week — the first time someone has died from having COVID-19 twice.
Immunocompromised people — such as the Dutch woman, who had cancer and was receiving chemotherapy treatment — may be at higher risk for reinfection, according to the Times. For others, the first infection could be too mild to provoke strong immunity against the coronavirus. Alternatively, a person who has immunity from their first infection could get a large dose of the virus later, which could make them sick again before their immune system has a chance to kick into gear to prevent reinfection.
Because reinfection is possible, no one should act as if they’re invincible after recovering from COVID-19. “Individuals who have tested positive for SARS-CoV-2 should continue to take serious precautions when it comes to the virus, including social distancing, wearing face masks, and handwashing,” Mark Pandori, a professor of pathology at the University of Nevada, Reno School of Medicine and co-author of the case study about the 25-year-old, said in a statement.
Reinfection also means natural herd immunity is probably not a viable strategy for thwarting the pandemic. Herd immunity is the concept that only part of a population needs to be immune to an infectious disease to stop outbreaks of that disease. Herd immunity can occur naturally through spread of the virus or through vaccination. But if some individuals get sick again after being infected once, natural herd immunity is flawed. “[W]e cannot rely on immunity acquired by natural infection to confer herd immunity; not only is this strategy lethal for many but also it is not effective,” Akiko Iwasaki, an immunobiologist at the Yale School of Medicine, wrote in an editorial accompanying the case study.
Even without reinfection, relying on natural herd immunity is a bad idea. For most infectious diseases, about 70 to 90 percent of a population needs to be immune to reach herd immunity. But only about 10 to 15 percent of U.S. residents are immune to the coronavirus, according to the New York Times. Because more than 214,000 people have already died from COVID-19 in the U.S. and the country is nowhere near achieving herd immunity, we can expect many more deaths if the government opts for this approach — an approach many scientists have advocated that we steer clear of.
The better way to reach herd immunity is through a vaccine. Although people can get reinfected with COVID-19, this shouldn’t make a vaccine any less effective. Vaccines can be designed to grant longer-lasting and stronger immunity than the body does naturally in response to infection. Iwasaki told NPR, “The good thing about a vaccine… is that it can induce much better immunity.”