“Taking into consideration the special circumstances surrounding the buildings concerned, the department has no choice but to resort to this exceptional measure,” a Department of Health spokesman said.

Authorities also ordered thousands of residents in the wider neighbourhood, home to many families from the city’s ethnic minority communities, to undergo mandatory testing from Saturday.

Residents and health experts complained that it might be too late for the testing, to be completed in three days and covering at least 20 buildings in each of which a minimum of one infection has been confirmed.

The mandatory screening was ordered as the city on Friday confirmed 38 new infections, of which 14 were untraceable. Three of the latest cases were imported by arrivals from Switzerland, Indonesia and the Philippines. More than 40 preliminary-positive cases were also recorded, many of them close contacts of cases already identified in Yau Tsim Mong.

“We hope to conduct small-district mandatory testing in order to achieve the goal of zero infections in the district,” Secretary for Food and Health Sophia Chan Siu-chee said of the campaign to stem infections in the crowded area bordering Nathan Road, Jordan Road, Canton Road, Ferry Street and Kansu Street.

Authorities first issued a mandatory testing order for 26 Reclamation Street on January 8, then extended it to cover numbers 20, 22 and 24 four days later after more cases were identified in the four interconnected blocks.

About 90 residents were evacuated two days later on Thursday after midnight and experts were sent in to inspect the sewage systems of the buildings to determine whether the infections were spreading through them.

At least some of the infections were connected to construction sites already hit with outbreaks, said Dr Chuang Shuk-kwan, head of the Centre for Health Protection’s communicable disease branch.

“Many of the construction workers work at sites such as the Lam Tin-Tseung Kwan O Tunnel or the Central Kowloon Route, and many of them live in large families of seven to eight people, which is why we are seeing a chain of transmission from the construction sites to the tenement buildings,” she said.

Locals and experts welcomed the screening but expressed frustration the step was not taken sooner.

“It’s a well-known fact that this district contains many tenement buildings with many residents and construction workers living in subdivided flats,” said a Ferry Street resident who refused to give her full name. “All residents in this district should undergo mandatory testing, not just residents in the 20 buildings that were listed.”

Construction worker Chand Rudra Kumar, who lives in a subdivided flat on Temple Street with his family, hoped authorities could do more to help members of ethnic minority groups overcome language barriers during the screening process.

“There is a significant community of Nepalese people living in the area around Reclamation Street,” he said. “I definitely think the government should be aware of this and distribute information accordingly.”

Chan offered reassurances that staff members who spoke Nepali, Urdu and other languages of minorities would be assisting health authorities in explaining the new measures to locals.

But Dr Leung Chi-chiu, a specialist in respiratory medicine, warned it would be impossible to achieve zero infections in a community with just one-off testing.

“If the government hopes to achieve zero infections, it will need to conduct tests repeatedly until no new case is recorded in a building over a period of time,” Leung said. “If more than one infection is identified in a building, authorities will need to conduct another round of testing a few days after the first one, or else it will be difficult to stop the transmission chain.”

Government intervention was probably too late to prevent the spread of the disease from buildings on Reclamation Street to other parts of the community, he warned.

Professor David Hui Shu-cheong, a government adviser on the pandemic, agreed screening should have been done earlier and suspected insufficient manpower had hindered the government.

Vincent Cheng Wing-shun, a lawmaker representing the Kowloon West constituency hoped the government would send experts to conduct testing on pipes to determine whether they presented a risk of virus transmission, as he believed plumbing in many subdivided flats was connected haphazardly.

Building surveyor Vincent Ho Kui-yip suspected toilets and drainage platforms were set in “inappropriate positions”, causing problems such as backflow and release of odour.

“My biggest concern is that the floor drains are directly connected to the large waste water pipe,” he told a morning radio programme. “I suspect there is no U-shape water trap to form a water seal. When the water seal is lost, it can cause odour in toilets.”

Some subdivided units had turned areas meant to be used as bedrooms or living rooms into toilets, making the placement of pipes “entirely wrong”, he added.

Apart from the mandatory screening, free and voluntary tests will be offered to other district residents through the Henry G Leong Yaumatei Community Centre and a mobile testing station set up at the Kwun Chung Sports Centre in Jordan. At least one mobile testing vehicle would also be deployed.

Meanwhile, a cluster at the construction site of the Central Kowloon Route – linked to the Reclamation Street tenement blocks – continued to grow, with four new cases emerging to take the total to 26.

Residents in two more buildings, Hiu Fung House at Fung Wah Estate in Chai Wan and 15-27A Pitt Street in Yau Ma Tei, which is outside the mandatory testing zone, will also be ordered to undergo screening after infections were recorded in two or more flats.

Authorities are still tracking down one household in Yan Shek House at Shek Yam Estate in Kwai Chung, where all residents living in flats numbered 9 were transferred into quarantine.

Hong Kong’s tally of confirmed Covid-19 cases stands at 9,452, with 161 related deaths.

Additional reporting by Rachel Yeo and Elizabeth Cheung