Dr Chuang Shuk-kwan of the Centre for Health Protection said authorities were “concerned” about possible transmission in the Nabe Urawa restaurant as an earlier confirmed case, a 32-year-old woman, had also been there around the same time, separated by a row of tables from the student.

“Transmission might have occurred in this restaurant, similar to what had happened in some other places,” Chuang said. “As we found in the previous waves of infections, hotpot could make transmission of viruses easier because of aerosol spread.”

How Omicron is spreading in Hong Kong’s fifth wave of coronavirus infections

The first hotpot cluster emerged back in February 2020, with the virus spreading among a family who shared a meal, infecting about a dozen people.

Government officers visited the Causeway Bay site on Wednesday afternoon to investigate. Chuang urged all patrons who had visited the restaurant last Tuesday evening to contact health authorities.

Such individuals will have to be tested or even quarantined if they had dined there at the same time as the student.

Health authorities on Wednesday confirmed 22 new Covid-19 cases, including a Penny’s Bay quarantine centre security guard who lived in Tuen Mun and whose source of infection remained unclear.

Four close contacts of previously reported cases were also confirmed as infected while the other 17 infections were imported.

The latest cases brought the city’s tally of confirmed infections to 13,002, with 213 related deaths. Thirty-one previously reported Covid-19 infections were confirmed as Omicron, taking the total number of variant cases to 310.

Other preliminary-positive cases included a 19-year-old student from Hong Kong Tang King Po College, and two more residents from Maple Gardens Phase Three in North Point where the discovery of vertical transmission triggered an evacuation.

A 48-year-old resident in the North Point block who works for investment bank Citic Securities was first confirmed as infected on Tuesday.

His 83-year-old mother, who lived in the same flat 6B with him, and a 32-year-old construction supervisor who stayed in another flat B three levels above them, later tested preliminary-positive.

“We agree there was vertical transmission from flats 6B to 9B,” government pandemic adviser Professor Yuen Kwok-yung said after inspecting the building with officials.

“The owner of unit 9B never poured water into the floor drain in his bathroom, and so the U-trap dried out.”

He said virus-laden air particles from lower floors could rise up through the building via a so-called chimney effect, and could easily enter flats where the U-trap had dried out.

Compounding the risks was the 32-year-old on the higher floor keeping his windows closed during external wall maintenance for the whole building, and turning on the extractor fan when in the bathroom.

That allowed air from the sewage pipe to be easily drawn indoors to contaminate the environment, Yuen said.

Not opening windows also meant the virus could easily accumulate and lead to high viral loads within a building, he added.

While Environmental Protection Department tests found that people on the ninth floor could smell smoke from the sixth one, they believed this route of transmission was less likely than through the U-trap.

Residents from 14 flat Bs in the building would need to be evacuated to reduce their infection risks, Yuen said.

Dr Albert Au Ka-wing, from the Centre for Health Protection, said 103 people, including 75 residents, were tested in a lockdown sparked by the first infection on Tuesday.

Neither of the latest cases was tested during the lockdown. The woman has been in quarantine since January 8 when her son was listed as a close contact of an infected colleague. The 32-year-old man sought medical help at Queen Mary Hospital after he developed symptoms on Sunday.

Hong Kong leader vows legal action if Cathay exploited quarantine rules

It is the second example of vertical transmission in a building reported by the authorities in four days. On Sunday, residents from 19 flats in Mei Sun Building, Tai Po, were evacuated for the same reason.

Yuen said seeing another instance of vertical infections within such a short period of time suggested that Omicron transmission was at a very strong level.

He said there was no need for Hongkongers to specifically wear the highly protective N95 masks recommended by medical experts in the United States and Canada.

“Wearing N95 everyday by everyone in Hong Kong is not a practical thing. It’s expensive and not that available. People will feel very breathless after talking for around 20 minutes,” Yuen said, responding to queries from the public.

“In Hong Kong, the number of cases with Omicron is still low.”

Yuen said Hongkongers worried about Omicron transmission could consider wearing two surgical masks at once to enhance the infiltration function.

He later told the Post a combination of a surgical mask with a cloth one might provide a better fit.

Separately, from Thursday, civil servants who are not involved in anti-epidemic work may be allowed to work from home if they can “meaningfully perform duties” while away from the office, according to an internal email seen by the Post. The arrangement will last until February 4.

HSBC, Hang Seng Bank and Bank of China will close their branches an hour earlier, at 4pm, in view of the latest epidemic situation. ICBC branches in Tuen Mun’s San Hui and Tai Hing areas will be closed until further notice from Thursday.

Coronavirus cases up 50 per cent, deaths stable: WHO

From Monday, the Silka Seaview Hotel Hong Kong in Yau Ma Tei will be used as a quarantine hotel for close contacts, adding 268 units. That brings the number of quarantine units to about 5,000 for close contacts to cope with the demand stemming from the current outbreak.

Hong Kong’s fifth wave of infections emerged at the end of December, after a number of Cathay Pacific aircrew members broke home isolation rules and visited restaurants and bars while infected with Omicron.