A man in the United States has caught coronavirus twice, with the second infection becoming far more dangerous than the first, doctors report.
The 25-year-old man was admitted in hospital for treatment after his lungs could not get enough oxygen into his body.
The man who has now recovered, had no known health problems or immune defects that would make him particularly vulnerable to the novel virus.
Reinfection remains rare.
The study in the Lancet Infectious Diseases raises questions about how much immunity can be built up to the virus.
He tested positive for the first time on 18 April. His first wave of symptoms including sore throat, cough, headache, nausea and diarrhea resolved by 27 April. The man tested negative on 9 and 26 May. On the 28 May he developed symptoms again including fever, headache, dizziness, cough, nausea and diarrhea. On 5 June tested positive for the second time and was found to by hypoxic (low blood oxygen) with shortness of breath.
The patient caught coronavirus twice, scientists said rather than the original infection becoming dormant.
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.
The findings signal that a previous infection may not necessarily protect against future infection, Dr Mark Pandori from the University of Nevada said, adding that this could have significant implications for our understanding of coronavirus immunity.
At present reinfection seems to be rare and there have been only a few examples out of the more than 27 million confirmed coronavirus cases.
Reinfection has been reported in Hong Kong, Belgium, and the Netherlands said they were no more serious than the first. A case in Ecuador mirrored the US case in being more severe. The person however did not require hospital treatment.
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