A man in the United States caught coronavirus twice, with the second infection becoming more dangerous than the first, doctors said.
The 25-year-old required hospital treatment after his lungs could not get enough oxygen into his body.
Reinfections remain rare and he has now recovered.
But the study in the Lancet Infectious Diseases raises questions about how much immunity can be built up to the virus.
The man from Nevada had no known health problems or immune defects that would make him particularly vulnerable to Covid.
Scientists say the patient caught coronavirus twice, rather than the original infection becoming dormant and then bouncing back. A comparison of the genetic codes of the virus taken during each bout of symptoms showed they were too distinct to be caused by the same infection.
“Our findings signal that a previous infection may not necessarily protect against future infection,” said Dr Mark Pandori, from the University of Nevada.
“The possibility of reinfections could have significant implications for our understanding of Covid-19 immunity.”
He first tested positive on April 18, but began to experience symptoms such as sore throat, cough, headache, nausea and diarrhoea since 25 March.
Twenty days later on May 9 and May 26, he tested negative for coronavirus. But two days later on May 28, he developed symptoms again, this time including fever, headache, dizziness, cough, nausea and diarrhoea.
On June 5, he was tested positive for coronavirus and suffered from hypoxic with shortness of breath.
Scientists warned people who have recovered should continue to follow guidelines around social distancing, face masks and hand washing.
So far, reinfection seems to be rare – there have been only a few examples out of more than 37 million confirmed cases.