A patient in the Netherlands and another in Belgium have been confirmed as having been reinfected with the coronavirus, Dutch national broadcaster NOS reported on Tuesday, citing virologists.

The news follows a report this week by researchers in Hong Kong about a man there who had been reinfected four and a half months after being declared recovered. 

The NOS cited virologist Marion Koopmans as saying the Dutch patient was an older person with a weakened immune system. She said that cases where people have been sick with the virus a long time and it then flares up are better known.

But a true reinfection, as in the Dutch, Belgian and Hong Kong cases, requires genetic testing of the virus in both the first and second infection to see whether the two instances of the virus differ slightly.

Koopmans, an adviser to the Dutch government, said reinfections had been expected.

"That someone would pop up with a reinfection, it doesn't make me nervous," she said. "We have to see whether it happens often."

The Belgian patient had mild symptoms, the NOS cited virologist Marc Van Ranst as saying. But "it's not good news," he added.

He said the case shows antibodies the patient developed during the first exposure were not enough to prevent a second case with the slightly different variant of the virus.

He said it is not clear whether such cases are rare or whether there are "many more people who could have a reinfection after six or seven months."

But WHO Spokesperson Dr. Margaret Harris told the BBC that it is unclear whether this is likely to be a common occurrence.  

"What's important here is this is just one case out of more than 23 million, so while we did expect that it could happen it is not clear that this is something that is likely to happen to many people."

"We would expect that given the quality of the surveillance – and the study in Hong Kong shows what a high level of surveillance they are doing there – you would have expected to see many more cases if this was happening a lot," she said.

A 33-year-old Hong Kong man who recovered from COVID-19 was infected again four-and-a-half months later in the first documented instance of human reinfection, researchers at the University of Hong Kong said on Monday.

He was found to have contracted a different coronavirus strain from the one he had previously contracted and remained asymptomatic for the second infection.

Read more: Hong Kong researchers report first documented coronavirus reinfection

(With input from Reuters)