A Hong Kong man who recovered from Covid-19 was infected again four-and-a-half months later in the first documented instance of human re-infection, Reuters quoted researchers at the University of Hong Kong as saying on Monday.

The findings indicate the disease, which has killed more than 800,000 people worldwide, may continue to spread amongst the global population despite herd immunity, they said.

The 33-year-old male was cleared of Covid-19 and discharged from a hospital in April, but tested positive again after returning from Spain via Britain on Aug 15.

The patient had appeared to be previously healthy, researchers said in the paper, which was accepted by the international medical journal Clinical Infectious Diseases.

He was found to have contracted a different coronavirus strain from the one he had previously contracted and remained asymptomatic for the second infection.