A new case of COVID-19 reinfection in a young, immunocompetent patient has raised concerns that vaccine candidates won’t be as protective as hoped and herd immunity is not possible.

Medical, immunological and epidemiological experts from the University of Nevada in the US are reporting the first case of coronavirus reinfection in the country and the fifth in the world.

But unlike most other reported cases, the 25-year-old man developed more severe symptoms with the second infection, which was due to a significantly different genetic strain of the virus than the man’s first infection.

In The Lancet Infectious Diseases report, doctors describe how the young man was first diagnosed with COVID-19 via nasopharyngeal swab after presenting to a testing centre on 18 April.