The cases, in Belgium and the Netherlands, follow a report this week by researchers in Hong Kong about a man there who had been re-infected with a different strain of the virus four and a half months after being declared recovered – the first such re-infection to be documented.

That has fuelled fears about the effectiveness of potential vaccines against the virus, which has killed hundreds of thousands of people, though experts say there would need to be many more cases of re-infection for these to be justified.