Researchers at the University of Hong Kong on Monday said they have proved that a Hong Kong man was infected with Covid-19 for a second time – the world’s first such documented case.

The 33-year-old IT worker was cleared of Covid-19 and discharged from a hospital in April. He tested positive for the virus again after returning from Spain earlier this month.

Health officials at first were unsure if the man was a "persistent carrier" of the virus from his previous infection.

But the HKU research team said genetic sequencing showed the virus strains contracted by him in April and August were “clearly different”. The study has been accepted by the Clinical Infectious Diseases journal, the researchers said.

“Many believe that recovered Covid-19 patients have immunity against re-infection because most developed a serum neutralising antibody response. However, there is evidence that some patients have waning antibody level after a few months," the researchers said.

“Our findings suggest that the SARS-CoV-2 may persist in the global human population as is the case for other common cold-associated human coronaviruses, even if patients have acquired immunity via natural infection,” they said.

They said, therefore, patients who recovered after getting Covid-19 should also wear masks and maintain social distancing.

"Since the immunity can be short-lasting after natural infection, vaccination should also be considered for those with one episode of infection," they said.