The man spent four days in a restricted area of the airport due to a visa-related issue, testing positive during a pre-departure screening.

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According to a press statement by the Food and Health Bureau, all transit passengers, including from Taiwan, will have to present results of a Covid-19 test taken within 72 hours before boarding. The same has been required of passengers entering the city.

“This is to strengthen management of transit passengers and monitoring of the coronavirus,” the bureau said.

Hong Kong authorities will also ban visitors from five more countries to shut out Omicron cases, adding Saudi Arabia, South Korea, Ghana, Norway and Finland to the city’s high-risk Group A list, with the move to take effect on Sunday.

Meanwhile, two passengers who flew to Singapore from South Africa tested preliminary-positive for the Omicron variant. The island state has not yet been added to the Group A list.

Under the rules, non-residents cannot enter Hong Kong from Group A countries while city dwellers, who must be fully vaccinated, face a mandatory 21-day quarantine.

There are a total of 53 countries classified as Group A, 28 of which were added since the World Health Organization designated Omicron as a variant of concern last Friday.

Before Thursday’s announcements, University of Hong Kong (HKU) microbiologist Dr Ho Pak-leung called for tougher vaccination requirements for transit passengers, noting that the 38-year-old man, who was unvaccinated and asymptomatic, had moved freely in a large area of the airport.

Cathay Pacific’s chief financial officer Rebecca Sharpe on Wednesday said the airline was hoping to capitalise on luring transit passengers through Hong Kong.

The latest measures could cause such passengers to bypass the city for fear of testing positive for Covid-19 and having to be put into isolation.

With Cathay’s core business of commercial flights in disarray, the airline has been scrambling to mount as many flights as it is able to operate, however small the amount.

The carrier was forced to activate contingency plans as tighter, impending rules on aircrew loomed. Cathay cancelled 28 of 61 passenger flights and five of 15 cargo services on Thursday, according to the airport’s flight departure data.

Transit passengers worldwide typically have to be screened for Covid-19, depending on their point of departure, but most do not face the stringent standards set by Hong Kong.

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The city also confirmed six new imported Covid-19 infections on Thursday, taking the official tally to 12,445 cases, with 213 related deaths. The arrivals from Britain, Pakistan, Qatar, South Africa, the United States and Russia were all fully vaccinated and carrying mutated strains of the virus.

The passenger from Qatar is known to have visited Morocco, Egypt, Rwanda and the Democratic Republic of Congo during his travels.

He was staying in Hong Kong’s Tai Kok Tsui area before leaving on November 8. The government has ordered compulsory testing for people in areas of the city where he stayed or visited during his incubation period.

Health authorities also cordoned off On Kin House in Lam Tin for mandatory testing, after a 28-year-old woman who had travelled to the US tested preliminary-positive for Covid-19. She was found to be carrying the L452R mutation linked to several variants, including the Delta one.

The Centre for Health Protection, meanwhile, said the 38-year-old transit passenger from Qatar who tested positive on Sunday had first travelled from Papua New Guinea to Nigeria on November 15 before subsequently returning to the city.

A total of 16 airport workers have been identified as close contacts of the man and sent to quarantine. All 13 post-disinfection environmental samples collected from places the passenger had stayed at the airport tested negative for the virus.

The Airport Authority said on Thursday that transit and transfer passengers who had to lay over at the hub for more than 24 hours would be confined to guarded individual rooms in a restricted “orange zone” – designated for international travellers – while awaiting connecting flights. They would be escorted directly to their flights once they were allowed to board.

The authority said it would coordinate with carriers over the tightened measures.

Airlines which failed to fulfil requirements relating to transit passengers proving they are allowed entry to their final destination would be subject to a penalty.

Earlier on Thursday, an authority spokesman said there were currently no other transit passengers staying at the hub. Any passengers who are stuck must stay in designated areas overseen by security guards, a requirement met by the Qatar individual.

Transit passengers must stay within the orange zone, and are not allowed to enter the “green zone” maintained for mainland Chinese travellers, according to the authority.

Staff must also remain within their designated zones and wear protective gear.

It is uncertain if there were other transit passengers who may have interacted with the infected case in the designated zone. The Post has contacted the airport authority seeking further details.

The designated zone for transit passengers was established after dozens of travellers mistakenly allowed to board flights to other countries with travel restrictions last year ended up marooned, with one man staying in the airport for three months before finally entering Hong Kong.

But with the heavily mutated Omicron variant sparking tough new border controls across the world – including outright travel bans – local health experts have called for a further tightening of restrictions for transit passengers.

Separately, health authorities conducted genome sequencing analysis on samples collected from 11 patients linked to three clusters of imported cases. They had taken the same Emirates Airlines flight 384 to Hong Kong from Dubai via Bangkok on November 21.

Eight cases involved a family that travelled from Pakistan, one was a passenger who boarded the flight in Bangkok, while the remaining two were a couple that travelled from India.

The analysis showed that all 11 patients carried the Delta variant, and six of them had identical genetic sequences, suggesting they were epidemiologically linked.

“The Centre for Health Protection cannot rule out the possibility that the family cluster of eight cases may have spread the virus to other cases by personal contact and shared use of toilets in the aircraft,” it said in a statement on Thursday evening.

Fifty-seven passengers are currently quarantined at designated hotels, while four others who were identified as close contacts have been sent to the Penny’s Bay isolation facility.

Additional reporting by Zoe Low