Health care workers wearing protective gear take swab samples from people at a drive-through coronavirus testing center in Seattle.
Researchers have reported the United States' first confirmed case of coronavirus reinfection.
Writing in the journal Lancet Infectious Diseases, they say the 25-year-old man's symptoms were much more serious the second time around.
The BBC's James Gallagher reports.
There have been more than 37 million cases of coronavirus and only a handful of proven reinfections.
This patient in Nevada developed a cough in March and tested positive.
He fully recovered and had two negative tests in May.
But then he tested positive for COVID again.
This time, his symptoms were more severe and he needed oxygen therapy in hospital.
An analysis of the coronavirus' genetic code from each bout of COVID showed the patient did catch the virus twice.
The researchers said that people cannot assume that they are protected after an infection.
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