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 reinfection with the virus, researchers at the University of Hong Kong said on Monday.

The findings indicate the disease, which has killed over 800,000 people worldwide, may continue to spread globally despite herd immunity, they said.

The 33-year-old man 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 a paper that was accepted by the international medical journal Clinical Infectious Diseases.

David Hui Shu-cheong, a respiratory disease expert at the Chinese University of Hong Kong, said there's a lot of uncertainty about the virus, and he called on the recovered COVID-19 patients not to let their guard down and travel to high-risk countries

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

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The finding does not mean taking vaccines will be useless, said To Kai-Wang, one of the leading authors of the paper. "Immunity induced by vaccination can be different from those induced by natural infection," To said. "We will need to wait for the results of the vaccine trials to see how effective vaccines are."

World Health Organization epidemiologist Maria Van Kerkhove said on Monday that there was no need to jump to any conclusions in response to the Hong Kong case.

Studies tracking larger numbers of cases over time are needed to better understand the quality and durability of recovered patients' antibody response, she said.

Instances of people discharged from hospitals and testing positive again for COVID-19 infection have been reported on the mainland. However, in those cases it was not clear whether they had contracted the virus again after full recovery, as happened in the Hong Kong case.

READ MORE: 2 Chinese patients test positive months after virus recovery

David Hui Shu-cheong, a respiratory disease expert at the Chinese University of Hong Kong, said there's a lot of uncertainty about the virus, and he called on the recovered COVID-19 patients not to let their guard down and travel to high-risk countries.

Hong Kong's former secretary for food and health Ko Wing-man said the case tells people not pin too much hope on the herd immunity hypothesis. Ko said it remains important to take precautions.

In view of a "reassuring" declining trend in confirmed COVID-19 cases in the special administrative region, Hong Kong will relax social distancing rules starting on Friday, allowing dine-in service at restaurants until 9 pm and the removal of masks during outdoor exercise.

Lam Ching-choi, a specialist in pediatric and community medicine, suggested that people from all walks of life－but especially restaurant owners, schoolteachers and elderly caregivers－should be mentally prepared to coexist with the virus.

READ MORE: Mainland expert: More evidence needed to confirm reinfection