The head of the University of Hong Kong's Centre for Infection, Ho Pak-leung, said there is evidence to support a shortened quarantine for those receiving the Pfizer/BioNTech jab, with the risk of infection cut by 90 percent.

But infectious diseases expert Leung Chi-chiu said there is no proof that vaccination will shorten an incubation period.

He warned it is not the right time to relax the measure as Hong Kong is under the threat of more contagious mutant strains.

That came as health chief Sophia Chan told legislators that officials are studying how quarantine for fully vaccinated people could be shortened.

They could also be exempted from tests after getting a second jab.

But Leung responded: "Hong Kong is facing the risk of mutant strains spreading, and we are hoping the case number can be kept at zero. Under this circumstance anti-pandemic measures should be tightened.

"If a single case spreads into the community it would take a lot of effort to handle it."

Leung also said there is no evidence to prove that shortening isolation for vaccinated people is even feasible.

"No single kind of vaccine can be 100-percent guaranteed [to prevent] transmission after the jab, including the two types of vaccine used in Hong Kong, especially against the mutant Covid-19 strains."

He also said the incubation time could be very long regardless of vaccinations.

But Ho said the UK found people who have received the Pfizer/BioNTech vaccine could see their risk of infection reduced by 90 percent, and even if they were infected transmission of the virus to others would be reduced by 50 percent.

No such statistics are available for the mainland-made Sinovac vaccine, he added.