Two European patients were confirmed to have been re-infected with the coronavirus, according to regional public broadcasters, raising concerns about immunity.

The news follows a report this week by researchers in Hong Kong about a man there who had been re-infected four and a half months after recovering.

Broadcasters said on Tuesday a patient in the Netherlands and another in Belgium had also been re-infected with the virus.

Dutch broadcaster NOS cited virologist Marion Koopmans as saying the patient in the Netherlands was an older person with a weakened immune system. “That someone would pop up with a re-infection, it doesn’t make me nervous,” she said. “We have to see whether it happens often.”

The case in Hong Kong is the first lab-confirmed reinfection. Genetic sequencing by scientists at the University of Hong Kong established that the second episode, in an otherwise healthy young man, was caused by a slightly different strain. Researchers had hoped that the man’s immune system would still have recognised and fought off the virus at the second encounter.

Dr Kelvin Kai-Wang To and colleagues say people who have recovered from Covid-19 should not be assumed to be immune. They should still be offered vaccination, once it is available, and should also comply with mask-wearing and social distancing restrictions.

“Our findings suggest that Covid-19 may persist in the global human population, as is the case for other common-cold associated human coronaviruses, even if patients have acquired immunity via natural infection,” they said in a statement.