Professor Yuen Kwok-yung, chair professor of the Department of Microbiology at the University of Hong Kong, told China Daily on Tuesday in an exclusive interview that a re-infected patient can also be infectious, although maybe less so than when infected for the first time. He said that more studies are needed to determine the probability of reinfection.

The interview took place one day after an HKU research team, led by Yuen and two others, announced what they believed to be the world's first case of coronavirus reinfection, as a 33-year-old Hong Kong man contracted the virus again over four months later. The full report is due to be published in Clinical Infectious Diseases, an official journal of the Infectious Diseases Society of America.

The man, who returned from Spain via the United Kingdom, has been infected by a different coronavirus strain and remained asymptomatic during the second infection. The second strain matched the type prevalent in Europe in July and August.

Yuen said the case showed that the man was able to mount a very good immune response and rapidly control the second infection, perhaps because his immune system had been primed during his first infection.

The 73-year-old scientist said this new finding means that recovered COVID patients will also need vaccinations for a prolonged period of protection in their lifetimes.

The key to the research on reinfections and vaccinations lies in how fast a human's antibody level against the virus goes down, he noted.

People will probably need two vaccine injections, Yuen said. "It is not unusual to receive two doses. It is not unusual to have falling immunity as time goes by.

"Our conclusion at this stage is that we have to be careful. ... You still have to wear a mask. You still have to practice good hygiene. You still need to have social distancing," he advised.

Yuen expects Hong Kong people to be able to get vaccines within the next six to 12 months, including ones made on the Chinese mainland and which are entering the third phase of clinical trials.

Once data can show that a vaccine is safe and effective, it will be time to launch a comprehensive vaccination campaign in the city, with priorities given to front-line healthcare workers and vulnerable groups like the elderly, he said.

HKU's Faculty of Medicine is developing two types of vaccines against COVID-19. Yuen said related animal experiments have all finished and the results look "very promising".

There have been sporadic cases reported in Hong Kong and on the mainland about recovered patients testing positive for the coronavirus again after they were discharged from hospitals. Health experts attributed these results to virus leftovers, which are not believed to be contagious.