The world is in shock as researchers in Hong Kong say they have "proved" the world's first known documented case of a human catching coronavirus twice despite a successful recovery.

The case involved a 33-year-old man who was initially infected in April and recovered with only mild symptoms. Researchers from the Department of Microbiology at the University of Hong Kong said that he has been reinfected within just under five months.

The man's second infection was detected after an airport screening after his return to Hong Kong from Spain earlier this month.

Researchers described the strains as "clearly different".

The scientists hypothesised the asymptomatic symptom might indicate "subsequent infections may be milder".

"An apparently young and healthy patient had a second case of COVID-19 infection which was diagnosed 4.5 months after the first episode," University of Hong Kong researchers said in a statement.

The findings are equally alarming because it suggests the threat of reinfection to coronavirus exists "even if patients have acquired immunity via natural infection or via vaccination," they said.

"Many believe that recovered COVID-19 patients have immunity against reinfection because most developed a serum neutralising antibody response.

"However, there is evidence that some patients have waning antibody level after a few months.

"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.

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

The IT worker was asymptomatic initially but genomic sequencing shows that he has been infected by two virus strains.

The second strain was a close match to the one travelling across Europes in July and August.

What the findings mean for potential vaccines is yet to be seen, but the World Health Organisation's technical lead on COVID-19 Maria von Kerkhove said that there isn't enough data to understand the implications and urged people to not "jump to any conclusions".

"It's very important that we document this and that any countries that do this, if sequencing can be done, that would be very, very, helpful," she said.

"But we need to not jump to any conclusions, to say even if this is the first documented case of reinfection, it is possible, of course."

More than 24 million people have been infected worldwide with coronavirus.