Health officials have said two patients from the Netherlands and Belgium have been reinfected with Covid-19, raising concerns about people's immunity to the virus.
It comes after the microbiology department at the University of Hong Kong (UHK) revealed on Monday that they had documented the world's first official case of coronavirus reinfection.
The scientists at UHK used genome sequencing to show the 33-year-old man from Hong Kong contracted Covid-19 strains in April and August that were "clearly different"
Belgian virologist Marc Van Ranst said the case of Covid-19 reinfection in Belgium was of a woman who had was infected for the first time in March and then again in June.
He told Reuters: "We don't know if there will be a large number. I think probably not, but we will have to see.
"Perhaps a vaccine will need to be repeated every year, or within two or three years. It seems clear though that we won't have something that works for, say, 10 years."
Mr Van Ranst added that the woman's symptoms were relatively mild, and her body may not have created enough antibodies to prevent reinfection.
The National Institute for Public Health in the Netherlands said on Tuesday that it had also observed a Dutch case of reinfection.
Virologist Marion Koopmans told the Dutch broadcaster NOS that the patient 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."
Europe starts to ease itself out of Coronavirus lockdown
Dr Margaret Harris from the World Health Organisation (WHO) told BBC Breakfast the discovery of reinfections was an "important piece of science" and that the health organisation has been tracking anecdotal reports of possible reinfection or infections with a different strain.
“Prior to this, it wasn’t clear whether the tests were perfect – you’ve got a negative when someone was still infected with the same virus," said Dr Harris.
“So this is the first time we have seen very clearly two different versions of the same coronavirus.
“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, because 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.
“But what it also tells us is what we long suspected, that we don’t know enough about how long immunity lasts or whether it lasts a long time in most people or many people, or not many people so all of those questions are still open.”
When asked if the news indicated a second spike, Dr Harris responded: “That doesn’t really necessarily indicate whether or not you would have a second spike but what it does indicate is this idea that many people had was like ‘just let it wash over us, it will be fine’, was never a wise choice.”