In a statement, the government urged the public, including those fully inoculated with the German-made BioNTech vaccine, to get a booster shot, which can be taken if at least six months have passed since the second jab.

“Eligible persons should take the third vaccine doses as soon as possible to strengthen [the vaccine’s protective effect] and build an immunity barrier in the community,” the statement said.

Those who need a booster to travel can request it just three months after their second jab, but must be assessed by staff at a government vaccination centre.

Adolescents aged 12 to 17, meanwhile, will be allowed to take a second dose of BioNTech under the new arrangement. The age group had previously been limited to one dose amid fears of myocarditis, a typically temporary condition involving inflammation of tissues around the heart.

The new guidelines were released as Hong Kong confirmed five new Covid-19 infections on Friday – four imported and one transmitted at the airport – taking the tally to 12,555 cases, with 213 related deaths.

Among the 25 or so preliminary-positive cases were five karate athletes and a Cathay Pacific cabin crew member, who returned to Hong Kong from Sydney via flight CX138 on Friday.

The athletes arrived back in the city via Seoul on Wednesday evening, after competing in the Asian Karate Championships in Kazakhstan. Thirteen athletes, coaches and staff who travelled with them were sent to the Penny’s Bay quarantine facility.

Cathay said in a statement that the crew member tested positive on arrival at the airport and had not entered the community.

Health authorities also revealed that seven previously confirmed imported cases were of the Omicron variant following whole genome sequencing analysis.

Hong Kong has now confirmed a total of 41 Omicron cases – all imported – since the first such infection was recorded in late November.

Among Friday’s tally of confirmed Covid-19 cases was a 64-year-old cleaner who worked in an airport toilet used by arrivals who had tested preliminary-positive. He was classified as an import-related case.

The four other infections were imported from Germany, Ghana, Nigeria and Australia.

The cleaner was suspected to be infected with the more transmissible Omicron as his viral samples carried two key mutations linked to the variant, pending genome sequencing.

As a precautionary measure, health authorities will quarantine 14 of the cleaner’s close contacts and about 40 others linked to those contacts.

Hong Kong panel recommends booster for fully vaccinated BioNTech users

For booking third doses, a 24-hour website will open from 8am on January 1, with seven vaccination centres devoted to providing BioNTech jabs, while three others will continue to administer Sinovac.

Same-day tickets will be available, meaning some of those eligible will be able to get their jab on the day of booking.

Nine of the 10 centres will extend their opening hours from 8am to 8pm. Only the hours at Gleneagles Hospital Hong Kong will remain unchanged.

Professor David Hui Shu-cheong, chairman of the scientific committee on emerging and zoonotic diseases, suggested residents opt for the BioNTech vaccine when getting a third dose.

He told a radio programme that local data showed three BioNTech jabs provided 160 units of neutralising antibodies against Omicron while three Sinovac jabs offered only 10. At least 26 units are needed for sufficient protection.

The same data showed the protective antibody level of two Sinovac jabs combined with one BioNTech booster reached 60 units.

Earlier on Friday, no coronavirus cases were found during an overnight lockdown of the cleaner’s residential building at Siu Hei Court in Tuen Mun.

Residents were allowed to begin leaving at 6.15am, authorities said, after more than 1,200 were tested for Covid-19 in a bid to prevent a potential community breakout of the swiftly emerging strain.

Hui said the worker had taken two jabs of Sinovac but that his job meant his risk of exposure to the virus was high, particularly given that his job involved cleaning toilets.

He urged airport staff to get a booster shot as soon as possible.

A spokesman for the Airport Authority Hong Kong (AAHK) on Thursday night confirmed that the patient, who had no recent travel history, was a cleaner working in Temporary Specimen Collection Centre One in the Orange Zone of the T1 Midfield Concourse. He last worked there on Wednesday.

According to the airport’s contractor guidelines, frontline staff should avoid being in the toilets at the same time as any arriving passengers.

“AAHK will require the relevant contractor to strengthen measures and control, to ensure that all staff comply with relevant regulations on toilet cleaning,” he said.

Meanwhile, the government added Serbia and the French territory of Reunion to its Group A “high-risk” list as part of its heightened efforts to keep Omicron out of the local community.

Non-residents will be barred from entering the city, while residents are required to undergo 21 days of hotel quarantine starting on Monday.