A Nevada man appears to be the nation’s first confirmed case of COVID-19 reinfection, researchers say.
The case is detailed in an online preprint, a study that has not yet been peer reviewed before officially being published.
Full coverage of the coronavirus outbreak
The case involves a 25-year-old man living in Reno, Nevada, who first tested positive for COVID-19 in mid-April. He recovered, but got sick again in late May. The second time around, his illness was more severe, the case report said.
Researchers reported that genetic sequencing of the virus revealed that he had been infected with a slightly different strain, indicating a true reinfection.
It’s still unclear why the patient was reinfected. The cause could lie in his immune system, the virus itself, or a combination of the two.
Mark Pandori, director the Nevada State Public Health Laboratory and one of the authors of the report, stressed that reinfection with the coronavirus appears to be rare. This is the first instance reported in the U.S. among the nation’s nearly 6 million cases so far, and “may not be generalizable” to the public, Pandori said.
Still, he urged caution. “If you’ve had it, you can’t necessarily be considered invulnerable to the infection” again, said Pandori, who is also an associate professor of pathology and lab medicine at the University of Nevada, Reno School of Medicine.
“The evidence so far suggests that if you’ve been infected and recovered, then you’re protected for some period of time,” Dr. Ashish Jha, director of the Harvard Global Health Institute, said. “We don’t know how long, and we’re going to find individual cases of people for whom that’s not true.”
Indeed, on Monday, a case of COVID-19 reinfection was reported in Hong Kong — the first such confirmation of reinfection during the pandemic. Two European patients, one in Belgium and one in the Netherlands, were also reported this week to have been reinfected with the virus.
But in those instances, the patients did not get sick the second time around, or they developed much milder forms of the illness than their first infection.
“You’d expect the second time around people to have much milder or ideally no symptoms,” Jha said. That’s because the immune system should be able to mount a more robust response, and the Hong Kong case was “completely consistent with that.”
Download the NBC News app for full coverage of the coronavirus outbreak
In the Nevada case, however, the man got sicker the second time. When he was first infected, he had typical symptoms for the coronavirus: headache, cough, sore throat, nausea and diarrhea. Within about 10 days, the symptoms cleared up, and he tested negative for the virus.
But a month later, on May 28, he started feeling sick again, experiencing dizziness as well as the previous symptoms.
The illness did not clear up quickly this time. Within a week, his blood oxygen level fell dangerously low. He needed help breathing, and was hospitalized. Once again, the man tested positive for SARS-CoV-2, the virus that causes COVID-19.
“That’s very concerning,” Dr. William Schaffner, an infectious disease expert at Vanderbilt University Medical Center in Nashville, said.
“If this sort of reinfection is common, then we have to worry about how strong the protection will be that we get vaccines,” Schaffner said.
If reinfection were common, however, “we would have seen it,” Jha said. “There’s so much disease in our country.”
Dr. Michael Mina, an epidemiologist at the Harvard University T.H. Chan School of Public Health, said the findings should not cause alarm.
“The vast majority of people who do get infected a second time will not be sick, and will not end up in the hospital,” Mina said. “It could be that the person just didn’t have a particularly bad first infection didn’t develop as good of an immune response.”
The case illustrates the need for continued protective practices, such as wearing masks, washing hands and maintaining physical distances, to reduce the risk for second infections.
“We need to maintain all of the behaviors that allow us to keep the virus at bay,” Pandori said.
Follow NBC HEALTH on Twitter & Facebook.