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."