The case 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.

Scientists said that while known incidences of reinfection appear rare - and the 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.

The World Health Organization said that the US case underlined what was unknown about immunity.

This also really is an argument against what some have been advocating, and that’s building naturally what is called herd immunity, a WTO spokesperson said.