The first study to investigate the case of a person in the U.S. who contracted COVID-19 twice found reinfection can occur swiftly and the second bout of illness can be more severe.

The research, published in the Lancet medical journal, examined the case of a 25-year-old man living in Nevada who became infected with two different genetic variants of the SARS-CoV-2 virus in fewer than two months. He tested negative twice in between, meaning he’s unlikely to have suffered a single prolonged infection.

The findings come as President Donald Trump says he’s immune to the virus after a single encounter. Any new findings on resistance can also have implications for a vaccine as drug makers race toward the finish line.

The degree of protective immunity after a COVID-19 infection is one of the pandemic’s great unknowns.

So far a handful of reinfection cases have been recorded since the start of the outbreak late last year. One patient in Ecuador also suffered a worse bout of illness the second time around, and an elderly woman in the Netherlands died after testing positive a second time. It’s also possible people with no symptoms could be infected multiple times without knowing it.

“There are still many unknowns,” said Mark Pandori, director of the Nevada State Public Health Laboratory and lead author of the study. “Our findings signal that a previous SARS-CoV-2 infection may not necessarily protect against future infection.”