A man from the US state of Nevada has become one of only a few people in the world to have caught COVID-19 twice.
The 25-year-old, who has not been publicly identified, had no known health conditions or immune problems that would have made him particularly vulnerable.
Doctors said the man needed hospital treatment after his lungs deteriorated during the second infection, which was much worse than the first.
According to the study in The Lancet Infectious Diseases journal, the man experienced initial symptoms – a sore throat, cough, headache, nausea and diarrhoea – on 25 March.
He tested positive for the first time on 18 April and his symptoms were resolved by 27 April.
On 9 and 26 May he tested negative, but on 28 May he developed symptoms again – this time these included fever, headache, dizziness, cough, nausea and diarrhoea.
On 5 June, he tested positive for the second time and went on to suffer low blood oxygen and shortness of breath.
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Scientists said that the genetic codes of the two illnesses were different, meaning it was not a case of the first infection being dormant and then reappearing.
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The study said the case was the first known COVID-19 reinfection in North America, with other single cases reported in Hong Kong, the Netherlands, Ecuador, and Belgium.
The patients in those places showed no increase in symptom severity the second time, with the exception of the patient in Ecuador.
It was not clear what had caused the second infection in Nevada.
The study suggested one possibility was a “very high dose of virus (which) might have led to the second instance of infection and induced a more severe disease”.
Another suggestion was that reinfection was caused by a “version of the virus that was more virulent, or more virulent in this patient’s context”.
A third possibility was a “mechanism of antibody-dependent enhancement… a means by which specific Fc-bearing immune cells become infected with virus by binding to specific antibodies”.
The study’s authors said: “Previous exposure to SARS-CoV-2 might not guarantee total immunity in all cases.
“All individuals, whether previously diagnosed with COVID-19 or not, should take identical precautions to avoid infection with SARS-CoV-2.
“The implications of reinfections could be relevant for vaccine development and application.”
Reinfections also have implications for concepts such as herd immunity.
It had been assumed that the body would learn to fight the virus during an initial infection, meaning that later infections would be milder or even without symptoms.
But it also shows that, even after almost 38 million confirmed cases and more than one million deaths from the virus worldwide, scientists are still learning about COVID immunity.