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, Reuters quoted scientists as saying 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.