The case of a man in the United States infected twice with Covid-19 shows there is much yet to learn about immune responses and also raises questions over vaccination, scientists said on Tuesday.
The 25-year-old from Reno, Nevada, tested positive in April after showing mild symptoms, then got sick again in late May with a more serious bout, according to a case report in the Lancet Infectious Diseases medical journal.
The report was published just hours after US President Donald Trump, who was infected with Covid-19 and hospitalised earlier this month, said he believes he now has immunity and felt "so powerful".
Scientists said that while known incidences of reinfection appear rare - and the Nevada man has now recovered - cases like his were worrying. Other isolated cases of reinfection have been reported around the world, including in Asia and Europe.
In the Netherlands, the National Institute for Public Health confirmed on Tuesday that an 89-year-old Dutch woman, also sick with a rare form of bone marrow cancer, had recently died after contracting Covid-19 for a second time.
Dutch media said this was the first known case worldwide of a death after SARS-CoV-2 coronavirus reinfection.
'IMPLICATIONS FOR VACCINATION'
"It is becoming increasingly clear that reinfections are possible, but we can't yet know how common this will be," said Simon Clarke, a microbiology expert at Britain's Reading University.
"If people can be reinfected easily, it could also have implications for vaccination programmes as well as our understanding of when and how the pandemic will end."
The Nevada patient's doctors, who first reported the case in a non peer-reviewed paper in August, said sophisticated testing showed that the virus strains associated with each bout of infection were genetically different.
"These findings reinforce the point that we still do not know enough about the immune response to this infection," said Paul Hunter, a professor in medicine at Britain's University of East Anglia.
Brendan Wren, a professor of vaccinology at the London School of Hygiene & Tropical Medicine, said the Nevada case was the fifth confirmed example of reinfection worldwide.
"The demonstration that it is possible to be reinfected by SARS-CoV-2 may suggest that a Covid-19 vaccine may not be totally protective," he said. "However, given the (more than) 40 million cases worldwide, these small examples of reinfection are tiny and should not deter efforts to develop vaccines."
World Health Organization spokesman Tarik Jasarevic concurred that the US case underlined what was unknown about immunity. "And this also really is an argument against what some have been advocating, and that's building naturally what is called herd immunity. Because we don't know," he told a briefing.