In a bid to further strengthen the city’s defences against Omicron and potential future variants, the Centre for Health Protection’s joint scientific committee also recommended a second jab for adolescents aged 12 to 17 who were previously asked to take one dose of the German-made BioNTech vaccine.

An interval of 12 weeks between the two shots will be required for that age group to reduce the chances of myocarditis, a typically temporary condition involving inflammation of tissues around the heart.

Chief Executive Carrie Lam Cheng Yuet-ngor last week said the government was aiming to expand the booster shot programme in early January to all residents who had previously received the BioNTech vaccine.

Hui noted people would be able to choose which vaccine to use for their third shot and that the recommendation also applied to women who were pregnant or breastfeeding. As mainland Chinese drug firm Sinovac has stated pregnant and lactating women should not receive its product, they could only take BioNTech shots.

Residents in need of extra protection before travelling to high-risk countries for work or study will only need to wait three months from their second shot.

Hui said a six-month interval between the second and third dose for the public would be good enough in light of the city’s situation.

“Currently there is no transmission of Omicron in Hong Kong, the situation is not as urgent as overseas,” Hui said. “And even if you have received a very strong vaccine, say the BioNTech one, the antibody level will also drop when time goes by.”

A joint study by the University of Hong Kong and Chinese University, announced on Thursday, found a third dose of the BioNTech vaccine induced “protective levels” of antibody against Omicron for those who had either taken the German-made jab or the Sinovac product for their first two shots. But those who take three doses of the Sinovac vaccine will not have adequate levels of protective antibody.

“Whether you have taken the BioNTech or Sinovac vaccine for your first two doses, the BioNTech vaccine is recommended for the third shot. But we will respect recipients’ choices,” said Hui, who co-led the study with Professor Malik Peiris from HKU.

Centre controller Dr Edwin Tsui Lok-kin said the threat to Hong Kong from Omicron was growing as more imported cases of the variant were being found. In the past few weeks, he said, the proportion of variant cases among imported infections had grown to about 80 per cent.

Health authorities on Thursday confirmed two new Covid-19 cases, both imported. Both carried the N501Y mutation, which is linked to the Omicron variant. The latest cases brought the city’s tally to 12,550, with 213 related deaths.

The airport cleaner, among several preliminary-positive cases identified on Thursday, was likely to have acquired the infection while working in the toilet solely used by preliminary-positive imported cases, health officials said. They classified it as an “imported-related case”.

The man, who had no recent travel history, was responsible for a concourse toilet used by people waiting to be transferred to hospital. He had stayed in its storeroom and had also been in the toilet at the same time as patients. The centre collected environmental samples at the storeroom, a changing room and a restroom used by the cleaner with test results pending.

The centre said workers should only perform cleaning after others had left the toilet, and they should not stay inside the storeroom when they were not working.

The airport worker had visited Tuen Mun Wu Hong Clinic, and another one at Tuen Mun Hospital, meaning 100 staff members would have to undergo testing.

The city’s booster shot programme, meanwhile, first opened in mid-November for high-risk groups including the elderly, the immunocompromised and workers in industries with high exposure to Covid-19, such as the health care sector.

The programme was extended in late November to include all those who had been fully vaccinated with the Sinovac shot, which has been found to be less effective at guarding against the virus than those employing mRNA technology.

So far, more than 331,000 people, or about 4.9 per cent of the eligible population, have received a third dose of vaccine.

Tsui said the current supply of vaccines would be sufficient to cover adults receiving their third shot and adolescents taking a second dose.

Experts said the recommendation of a second BioNTech shot for adolescents, who in September were asked to get just one dose to cut the risk of developing myocarditis, was due to the emergence of Omicron and having considered recent data.

Citing data from Canada, Professor Lau Yu-lung, chairman of the scientific committee on vaccine preventable diseases, said extending the interval between the first two doses for adolescents, from a current practice of 21 days for adults to at least three months, provided sufficient protection and was safer.

“If we extend the interval, there will be better protection and the chance of developing myocarditis could be reduced to one-tenth [of that of the 21-day practice],” he said.

As of December 14, about 73,000 adolescents had received one dose of vaccine.

Tsui said experts were still waiting for more data on how the Sinovac vaccine could reduce severe symptoms and deaths among young children, before making a decision on whether those aged under 12 could receive it.

The health minister has legally approved the vaccine for use on children as young as three, but the joint scientific committee has not yet issued recommendations on a vaccination strategy for those aged three years to 11.

The panel was also waiting for BioNTech to apply to further lower the vaccination age from the current 12 years, Tsui said.

Hong Kong will move four more places – Guadeloupe, Guernsey, the French part of Saint Martin and Venezuela – to its high-risk category from Sunday, following the detection of Omicron cases. Visitors will be banned from travelling to Hong Kong from those places, while only fully vaccinated residents will be allowed to fly in.