Two European patients are confirmed to have been re-infected with the coronavirus, according to regional public broadcasters, raising concerns about people's immunity to the virus as the world struggles to tame the pandemic.
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 being declared recovered.
That has fuelled fears about the effectiveness of potential vaccines against the virus, though experts say there would need to be many more cases of re-infection for these to be justified.
Broadcasters said on Tuesday a patient in the Netherlands and another in Belgium had also been re-infected with the virus that has led to hundreds of thousands of deaths worldwide and crippled the global economy.
Dutch broadcaster NOS cited virologist Marion Koopmans as saying the patient in the Netherlands 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 re-infection, 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 re-infections had been expected.
"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."
Dutch Health Ministry officials did not immediately respond to a request for comment.
Belgian virologist Marc Van Ranst told Belgian broadcaster VRT he had not been surprised by the Hong Kong re-infection.
"For us it was not news because we have also had such a case in Belgium," he told the Terzake programme.
The Belgian case was a woman who had contracted Covid-19 for a first time in the second week of March and for a second time in June.
"I think that in the coming days that we will see other similar stories ... These could be exceptions, but do exist and it's not just one," Van Ranst said. "It's not good news."
Van Ranst said that in cases such as the Belgian woman's in which the Covid-19 symptoms were relatively mild, the body may not have created enough antibodies to prevent a re-infection, although they might have helped limit the sickness.