In an interview with mainland media, Yuen from the University of Hong Kong said “mixing” vaccines from the two brands turned out to offer better protection compared to receiving three Sinovac jabs, according to a clinical trial his HKU team has conducted.

Yuen said the results proved high risk groups, including elderly people and those with weaker immunity, should take the BioNTech jab as a booster, and added that his team will soon publish a journal on the trial.

But he said that vaccination is a personal choice and there is nothing wrong with choosing all three doses to be Sinovac.

Yuen said he was worried about the current vaccination rate in Hong Kong -- 70 percent have taken at least one dose.

He said the top priority should be inoculating each and every citizen, and vaccine passports should be implemented to pump up the sluggish jab coverage.

"It is reasonable not only to protect people from serious illnesses and deaths but also not to deprive freedom. In fact, it ensures travel freely, so that it is a so-called civic responsibility," Yuen said

Yuen also said new vaccines are being developed based on the characteristics of Covid-19 coronavirus infecting the upper respiratory tract.

"One way is the nasal spray vaccine, which is scheduled to start its third phase of clinical trials overseas soon. The other is a combination of vaccines, and mixing the use of different vaccines can complement each other to achieve better immunity, and the adverse reaction is not prevalent," Yuen said.