Researchers for the first time have identified someone in the United States who was reinfected with the novel coronavirus, according to a study that has not yet been reviewed by outside experts.

The report, published online, describes a 25-year-old man living in Reno, Nevada, who tested positive for the virus in April after showing mild illness. He got sick again in late May and developed more severe COVID-19, the disease caused by the virus.

"This study likely represents a clear example of reinfection ... reinfections are possible — which we already knew, because immunity is never 100%," Kristian Anderson, professor of immunology and microbiology at Scripps Research in La Jolla, California, said in an emailed comment.

Cases of presumed reinfection have cropped up in other parts of the world, but questions have arisen about testing accuracy. Earlier this week, University of Hong Kong researchers reported details of a 33-year-old man who had recovered in April from a severe case of COVID-19 and was diagnosed four months later with a different strain of the virus.

Researchers at the University of Nevada, Reno, School of Medicine and the Nevada State Public Health Laboratory said they were able to show through sophisticated testing that the virus associated with each instance of the Reno man's infection represented genetically different strains.

They emphasized that reinfection with the virus is probably rare, but said the findings imply that initial exposure to the virus may not result in full immunity for everyone.

"We don't know at what frequency reinfections occur and how that might change over time," Anderson said. "Before we have broader studies illuminating these questions, we can't conclude what a single case of reinfection means for longevity and robustness of COVID-19 immunity and relevance for a future vaccine."