Hopes catching coronavirus leaves most immune were dealt a blow as a man in the US became the first person there to catch it twice.
The 25-year-old man caught a slightly different strain - and the second dose was so severe he ended up in hospital being treated with oxygen.
The authors of the study in the Lancet Infectious Diseases Journal said further research was required. They added the findings indicate previous exposure to the virus may not guarantee total immunity, and that all individuals should comply with control measures.
It is the fifth confirmation of reinfection worldwide, researchers said, with at least four other cases confirmed in Belgium, the Netherlands, Hong Kong, and Ecuador.