Hong Kong scientists are reporting the case of a healthy man in his 30s who became reinfected with coronavirus four and a half months after his first bout.
They say genome sequencing shows the two strains of the virus are “clearly different”, making it the world’s first proven case of reinfection.
The World Health Organization warns it is important not to jump to conclusions based on the case of one patient.
And experts say reinfections may be rare and not necessarily serious.
There have been more than 23 million cases of coronavirus infection around the world.
Those infected develop an immune response as their bodies fight off the virus which helps to protect them against it returning.
The strongest immune response has been found in the most seriously ill patients.
But it is still not clear how strong this protection or immunity is – or how long it lasts.
And the World Health Organization said larger studies over time of people who had previously had coronavirus were needed to find out more.