The man’s infection was the first since an airport lounge employee was confirmed to have the coronavirus on August 17.

Thousands of people will have to undergo mandatory testing.

The patient’s wife and five-year-old daughter both tested negative. The family of three live in Golden Glory Court Phase 1, in Tai Wai’s Golden Lion Garden. Testing of 400 residents there uncovered no new infections.

The daughter, who studies at Tai Wai Tung Wah Group of Hospitals Chan King Har Kindergarten in Sha Tin, developed a cough and runny nose on Monday while a classmate came down with a fever. Both were admitted to hospital and tested negative for the virus. School classes were suspended and about 30 children sent for three days of mandatory quarantine, where they would be tested for Covid-19.

“The risk of an outbreak does exist … we have yet to find the source of his infection,” said Dr Chuang Shuk-kwan, head of the Centre for Health Protection’s communicable disease branch.

Chuang said the man could have been infected by aircraft crew or through contact with contaminated goods, but she emphasised it would require a genome sequencing test to find the source.

“But since those who work with him have been put under regular testing, [an outbreak] should not happen on a large scale. We still do not know [the source], and whether people have come into contact with some goods with him,” she said.

He tested negative on September 29 but screening on Wednesday returned a positive result, although he displayed no symptoms. His viral load increased after admission to hospital, a reflection that his case was confirmed early, Cheung said.

The man mostly worked on the night shift, unloading and inspecting cargo from overseas. He spent most of his time working below aircraft, occasionally boarding cargo planes, Chuang said. He worked on flights to and from Europe and Asia, she added.

03:07

The Chinese ‘RoboDoc’ swabbing noses for Covid-19

The Chinese ‘RoboDoc’ swabbing noses for Covid-19

Chuang said the patient worked with various colleagues, sometimes eating and resting in a lounge, and had about 90 close contacts who would have to undergo mandatory quarantine. Several thousand people would be subject to mandatory testing, she said.

Government pandemic adviser Professor David Hui Shu-cheong said he believed more targeted measures could be implemented to address problems behind the worker’s infection after genome sequencing of his virus samples.

“We need to wait for [results of] the genome sequencing to see where the source of infection is, whether it was aircrew members from a certain flight. With this clue, it will be easier to know what happened,” Hui said, adding that transmission from foreign flight crews had occurred in the past.

In July, an airport porter’s infection was found to be linked to four previously imported cases from Russia, including two cargo flight crew members.

While airport workers are required to be fully vaccinated and undergo weekly testing, Hui said he believed screening could be increased to every three days. As the Delta variant’s incubation period is about four days, testing once a week could allow infections the chance to spread.

Hui said he did not think the one case would affect overall negotiations between Hong Kong and the mainland on reopening the border.

“Even the mainland and Macau will see cases sometimes despite their stringent measures,” Hui said. “There isn’t a completely foolproof border control mechanism.”

He added: “As long as we can prevent a community outbreak and continue to improve our measures, it should be fine.”

Officials also confirmed one imported case on Friday, involving a 29-year-old man, also carrying the L452R variant, who arrived from Britain. Six preliminary-positive cases were reported.

Meanwhile, the city’s leader asked residents to “bear with us, we will be working very, very hard” on resuming quarantine-free travel with the mainland, adding that it was more of a priority than reconnecting with the rest of the world.

Lam said further meetings with mainland officials and health experts would hopefully bring good news. However, she said she could not commit to a timeline because any reopening was “still under discussion”.

At a Legislative Council meeting, Undersecretary for Food and Health Dr Chui Tak-yi was subjected to heavy questioning by pro-establishment lawmakers over reopening the border.

“Is it three points or seven points. Just let us know!” said Priscilla Leung Mei-fun, referring to the undisclosed conditions laid down by the mainland side.

In reply, Chui only said that government departments were working on “concrete proposals” to satisfy those demands, which fell broadly into areas such as testing frequencies, contact tracing, and public hospitals’ discharge criteria for Covid-19 patients.

Carrie Lam gives no timetable for full reopening of border with mainland

As of Friday, the city’s total number of confirmed cases was 12,253, with 213 related deaths.

Separately, the British government announced on Thursday it would start recognising Hong Kong’s vaccination document and allow those from the city to take a post-arrival test to enter the country, beginning on Monday.