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 involves a 33-year-old man who was initially infected in April and recovered with only mild symptoms, but researchers from the Department of Microbiology at the University of Hong Kong say he has been reinfected less than five months later.
The man's second infection was detected after an airport screening upon his return to Hong Kong from Spain earlier this month.
The IT worker was asymptomatic but genomic sequencing shows he has been infected by two different virus strains.
The second strain was a close match to the one travelling across Europe in July and August.
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.
What implications that will have for potential vaccines is yet to be seen but the World Health Organisation's technical lead on Covid-19, Maria van Kerkhove, said overnight there isn't enough data to understand the implications and urged people not to "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."
The study has been accepted by the Clinical Infectious Diseases journal, the researchers said.
More than 24 million people have been infected worldwide with coronavirus, van Kerkhove said.
Patients who had already recovered from coronavirus should also wear masks and maintain social distancing, the researchers said.
Yale University Professor Akiko Iwasaki said the news was "no cause for alarm" and the case "is a textbook example of how immunity should work".
"Second infection was asymptomatic. While immunity was not enough to block reinfection, it protected the person from disease.
"Since reinfection can occur, herd immunity by natural infection is unlikely to eliminate #SARSCoV2. The only safe and effective way to achieve herd immunity is through vaccination."
Doctors have previously reported cases of presumed reinfection, but the case has been the first to be confirmed through scientific testing.
In July, experts told the New York Times that "it may be possible for the coronavirus to strike the same person twice, but it's highly unlikely that it would do so in such a short window or to make people sicker the second time".
In February, reports emerged of a woman in Japan that appeared to have caught the virus again after initially recovering.
There have also been similar reports of possible reinfection emerging from China but despite multiple reports of reinfection, some experts in April remained sceptical.