Hong Kong University scientists have reported the case of an apparently young, healthy patient being re-infected with Covid-19, four-and-a-half months after the first infection.

Using genome sequencing, they discovered that the second bout or virus was “clearly different” from the first infection rather than just a continuation or re-emergence of the original one.

Scientists know that people infected by coronavirus develop an immune response as their bodies fight off the virus, and this helps to protect them against it returning. But it is not clear how strong this protection is or how long it lasts.

If people can catch the virus more than once, then a vaccine may be required to protect many more people – although those who are most seriously ill have a greater immune response than those with mild symptoms.

Other coronaviruses, such as the common cold, do re-infect people so it’s not surprising that Sars-CoV-2 – the virus which causes Covid-19 – could do the same.

But the World Health Organization says it’s important not to jump to any conclusions based on the experience of one patient out of 23 million around the world.

The full research paper has not yet been published so some aspects of this case are not yet clear.

It appears that when this person was infected for the second time they had no symptoms – and the infection was picked up by airport screening tests, suggesting the patient wasn’t unwell. More population-based studies are needed before any firm conclusions on re-infection can be made.