DALLAS (NEXSTAR) — A 25-year-old Nevada man was reinfected with the coronavirus, according to research published earlier this week.

The case study published in the Lancet marks the first confirmed case of reinfection within the United States, according to National Public Radio. It’s the fifth confirmed reinfection case worldwide.

According to the research, the two infections occurred six weeks apart. The man’s first case happened April. He later tested negative for the virus in May. Researchers say he developed symptoms later that month and tested positive again in June. In the second case, his symptoms were severe enough for hospitalization.

NPR notes this is the second case of reinfection where a patient suffers more severe symptoms in their second battle with COVID-19.

“There are many reasons why a person might get sicker the second time around,” Akiko Iwasaki, a professor of immunobiology at Yale University, told NPR. “They may have been exposed to a lot higher levels of the virus the second time around.”

Dr. Anthony Fauci, the nation’s top infectious-disease expert, told CNN people who recover from COVID-19 are likely to be immune for a limited period of time, but there are cases emerging of people getting reinfected weeks or months later.

After his recent battle with coronavirus, President Donald Trump said he no longer needs to be concerned about infection because he’s now “immune.”

While there’s evidence that reinfection is unlikely for at least three months even for those with a mild case of COVID-19, very few diseases leave people completely immune for life, according to fact checkers from the Associated Press. Antibodies are only one piece of the body’s defenses, and they naturally wane over time.

“Certainly it’s presumptuous to say it’s a lifetime,” said Dr. Albert Ko, an infectious disease specialist and department chairman at the Yale School of Public Health.