“What we are facing is a once-in-a-century pandemic. We will need patience and the efforts of everyone to step out from this situation.”

Since the emergence of the virus locally a year ago, Hong Kong has experienced four Covid-19 waves and confirmed 10,282 infections, with 174 deaths.

Switzerland and the United Arab Emirates are to be the latest added to a list of high-risk nations, where people need a negative test result before boarding a flight to Hong Kong. The arrangement comes into effect on Tuesday.

The fourth wave, which began in late November, had lasted the longest and officials are grappling with lowering daily case numbers.

‘Ambush-style’ Covid-19 lockdown takes residents of 12 Yau Ma Tei buildings by surprise

University of Hong Kong infectious disease expert Professor Yuen Kwok-yung, a government pandemic adviser, explained why the current wave was proving difficult to beat.

“We are seeing a rebound of cases in the fourth wave, and it is very likely due to the construction sites clusters, which brought the virus into communities. Unfortunately, those communities are where many old buildings are located,” Yuen said after inspecting a North Point block hit by an outbreak.

Health officials had earlier revealed that workers at two large construction sites lived in tenement buildings in Yau Tsim Mong. Part of the district was locked down for emergency Covid-19 testing over the weekend, the first time the government had carried out such an operation.

“Apart from clusters of cases, the dense population and a higher number of old buildings and subdivided flats were believed to be reasons behind the transmission of the virus in that district,” Lam wrote.

Eleven blocks at 9-27 Pitt Street and the Shun Fung Building at 3 Tung On Street, a bustling section of Yau Tsim Mong, were the second area hit with a lockdown in an 11-hour operation that began at 7pm on Tuesday.

Lam said on her official Facebook page on Wednesday morning that the operation had achieved its targets.

“I’m happy that my colleagues have met the standards of the forced inspection within the restricted area overnight,” she said, adding the lockdown had met the goals as a confidential, area-specific and time-bound operation.

Lam also urged the public not to dismiss the results because only one case was identified. About 330 people were screened overnight.

“We cannot underestimate the impact of any single infection from the perspective of epidemic control, [as] the operation will offer residents comfort and reassurance,” she said.

Was it worth it? A look at whether city’s harshest coronavirus measure was effective

While the “ambush-style” lockdown appeared to be a new tactic in the fight against the virus, a respiratory medicine specialist urged the government to use more stringent standards to trigger testing in a building.

“If there is only one case in a building and some high-risk factors are identified, such as subdivided flats or poor environmental hygiene … a mandatory testing order could be issued regardless of whether the building is in a specified area [for stringent screening],” Dr Leung Chi-chiu said.

Currently, it requires cases in two unrelated flats to trigger a compulsory testing order in buildings outside such areas.

More efforts in maintaining environmental hygiene in high-risk buildings were also needed.

“We can’t just rely on testing … if the environment is persistently bad, there could still be outbreaks after testing,” Leung said.

Some people affected by the overnight lockdown were not convinced about the operation, while others were more accepting.

A 25-year-old construction worker, who declined to give his name, said the latest lockdown ran more smoothly than expected and that government workers came to his flat at around 10pm to get him tested.

“The lockdown overnight was quite easy. I think it’s much better than the two-day one in Jordan, because at least our work doesn’t get affected,” he said.

Yuen Kwai-wan, a security guard who works in a building that was spared from the nearby lockdown, said the cut-off area was arbitrarily drawn.

“I’m not convinced the small lockdown will be effective, because what if there’s Covid-19 traces in other places nearby?” he asked.

Yuen, in his 60s, reckoned the operation was “better than nothing” but also questioned whether it was just a stunt, given how quickly it was completed.

District councillors said the operation was ineffective and disruptive. Yau Tsim Mong District Council vice-chairman Andy Yu Tak-po of the opposition Civic Party said: “Some residents were coming home after doing their grocery shopping in the evening and found out on the spot that they were under lockdown.”

Fellow district councillor Lee Wai-fung said residents had been anxious about mandatory testing and lockdown orders since last week as they watched cases in the area mount.

“Communication could have been better [on Tuesday], because residents saw dozens of police officers around the neighbourhood but had no clue what was happening,” Lee said.

He added he wished the government would notify community representatives even 30 minutes in advance so they could explain the situation to residents and avoid fearmongering.