Scientists have reported the first known case of COVID-19 reinfection in the U.S. this week, nearly two months after the initial case of reinfection was identified in China. The news was delivered via a paper in the Lancet on Monday, detailing how a 25-year-old in Nevada tested positive for COVID-19 in April and then again in June, after testing negative twice.

A resident of Washoe County, the man received a positive COVID-19 result at a community testing event in April, where he reported symptoms such as sore throat, cough, headache and diarrhea. While quarantining, he reportedly recovered from the virus and ultimately tested negative two separate times in May.

But 48 days after the initial infection, he was admitted to the hospital with severe symptoms, including fever, dizziness, headache, nausea, diarrhea and cough. On top of these symptoms, he reported shortness of breath, which “required ongoing oxygen support,” the researchers note, likely due to pneumonia. His case is believed to be the fifth documented instance of COVID-19 reinfection worldwide, one of which — that of an 89-year-old Dutch woman — has proven fatal.

The authors say the case study further proves that reinfection is a possibility. “Previous exposure to SARS-CoV-2 does not necessarily translate to guaranteed total immunity,” they conclude, adding that the “implications of reinfections could be relevant for vaccine development.”

Although the Nevada man experienced more severe symptoms the second time, the first person to have documented reinfection — a 33-year-old in Hong Kong — was asymptomatic the second time. Dr. Amesh Adalja, a senior scholar at Johns Hopkins Center for Health Security, says that the 25-year-old Nevada man’s reaction was likely the result of a unique immune response.