The Sunday evening operation, which encompassed 432 units, came just hours after officials hinted they had not ruled out subjecting more districts to mandatory coronavirus testing schemes in coming days.

The government announced at 7pm that lockdown measures had been imposed on blocks 5 and 7 of Laguna City, after a run of infections in the two buildings. The lockdown, the fourth in the space of about a week, was launched on a day 53 new Covid-19 cases were confirmed citywide.

“From January 18 to 29, a total of 15 confirmed cases were recorded in blocks 5 and 7,” a government spokesman said. “After risk assessment, we believe there is a need to … ensure all people in the restricted area are tested mandatorily as a way to cut transmission chains in the area and dispel worries of residents there.”

Some Block 5 residents were evacuated and quarantined last Monday after vertical transmission of the virus was suspected. All residents of the two blocks must get tested by 2am on Monday, and remain in their flats until they receive the results. The operation was expected to end at around 7am on Monday.

People who had visited the blocks for two hours or more between Friday and Sunday are also subject to the testing order and deadline.

The two blocks had previously been issued mandatory testing orders, and nearly 1,200 people had been screened. The health minister, Professor Sophia Chan Siu-chee, visited the estate at around 10pm to check the operation.

Public health experts were split on the need for the operation. Chinese University’s Professor David Hui Shu-cheong, a pandemic adviser to the government, said the lockdown was justified as 15 cases in the two blocks was a “big number”.

But Dr Leung Chi-chiu, a specialist in respiratory medicine, questioned whether the measure was necessary if residents complied with the previous mandatory testing orders, a view echoed by district councillor William Li Wai-lam.

Elsewhere, a coronavirus cluster at a construction site of the new airport runway project continued to worsen while residents of another nine buildings faced mandatory testing.

Among Sunday’s cases, 44 were locally transmitted and included 17 without a clear source of infection. Nine were imported, with six from Indonesia, two from the Philippines and one from Russia.

The overall tally stood at 10,452 infections. More than 20 preliminary-positive cases were recorded.

Four more cases were linked to the runway site cluster, which has now grown to 15 patients, including two close contacts of infected workers.

Dr Chuang Shuk-kwan, head of the communicable disease branch of the Centre for Health Protection, said it would be worthwhile for construction workers to undergo regular testing. She added that it would be up to relevant government departments to decide whether to introduce such screening.

Construction workers ordered screened as Covid-19 cluster grows; another 28 cases logged

An on-site testing service, launched by the construction sector and supported by the Development Bureau, has tested more than 22,000 workers since late last month. A one-off free service was also offered to workers at community testing centres in January.

Chuang said construction workers used containers as changing rooms and each one was shared by up to eight people. She suggested adding more containers on site, space permitting, and to limit the number of workers using each one.

She also warned that an outbreak could affect any site as many workers lived in areas of Yau Ma Tei which were hardest hit by the coronavirus.

“It’s hard to say [how transmission is happening] but many construction workers, cleaners, security guards and restaurant workers live in Yau Ma Tei, so naturally if many of them work in construction sites there can be transmission,” she said.

Mandatory testing orders would also be issued for seven blocks in four specified testing areas in Jordan, Yau Ma Tei, Hung Hom and Sham Shui Po where a single infection in a building triggers mandatory testing.

Two untraceable sources were also identified in Jordan at Man Yuen Street’s Man Yuen Building, where sewage samples tested positive for the virus.

The city’s death toll rose to 181, after two men, aged 83 and 95, became the latest fatalities.

Earlier, writing on his blog, Chief Secretary Matthew Cheung Kin-chung said government departments had been asked to be flexible with manpower arrangements so civil servants could be mobilised at short notice to help with testing drives.

“Reaching zero infections is the government’s target. Based on the situation in Hong Kong, we will take accurate, decisive and strict prevention measures,” Cheung said.

“We do not rule out the possibility we will run more mandatory testing schemes of varying scales and using different modes, in different districts.”

His comments followed the completion of neighbourhood-level lockdowns in Jordan, Yau Ma Tei and North Point over the past week. The operations restricted movement in and out of designated areas to allow for comprehensive testing of residents in coronavirus-hit buildings.

While the 44-hour lockdown in Jordan affected a larger community, the ones in Yau Ma Tei and North Point targeted only a few buildings and were completed overnight.

The unprecedented move, which uncovered 13 new infections in Jordan, one in Yau Ma Tei and none in North Point, was hailed as a success by officials including Chief Executive Carrie Lam Cheng Yuet-ngor.

On her blog, Chan, the secretary for food and health, said mandatory testing orders and lockdowns had been effective in finding hidden infections in the community.

She said the experience gained under the first three lockdowns would allow authorities to identify cases in a shorter amount of time going forward.

Health officials have also recently conducted checks of residents living in buildings hit with previous mandatory testing orders in Sham Shui Po and North Point, she noted, fining 29 people who had violated the orders HK$5,000 each.

“I hope these short inconveniences can finally break the chains of transmission and allay residents’ concerns so they can return to normal life,” Chan said.

Meanwhile, Secretary for Labour and Welfare Dr Law Chi-kwong said on his blog that coming vaccinations and the increase in Covid-19 testing capacity would help speed up the resumption of visits at care homes for the elderly, which could improve residents’ mental health.