“Even though they have separate toilets and kitchens, the pipes and drainage may be connected ... it may also be related to shared corridors or stairs. It’s possible the infection may have spread this way,” she said.

The city’s overall tally of Covid-19 cases now stands at 9,797, with 166 related deaths.

The latest fatality was an 88-year-old woman with a chronic illness who died in Princess Margaret Hospital on Tuesday evening.

All of Wednesday’s caseload involved locally transmitted infections and 37 were classified as untraceable.

Hong Kong authorities have been running a mandatory-testing campaign in Yau Tsim Mong that includes establishing designated areas of compulsory screening around new outbreaks.

Officials on Wednesday said new mandatory-testing orders would be issued for eight buildings, including three in the Yau Tsim Mong designated area and one in the Sham Shui Po version.

Outside the designated areas, compulsory testing will also be required at Sheung Hong House in Upper Ngau Tau Kok Estate; Man Yiu Building on 25-47 Man Wai Street in Yau Ma Tei; 7 Waterloo Road in Yau Ma Tei; and Kornhill Block N in Quarry Bay.

Authorities revealed their district-level approach to mandatory testing last Friday, with the first designated area covering Nathan Road, Jordan Road, Canton Road, Ferry Street and Kansu Street in Yau Tsim Mong district. Each building there that had recorded at least one infection would fall under the tougher testing regime.

Health officials on Tuesday stepped up the system by declaring as higher risk a smaller “core area” within the wider zone. The core area covers about 70 buildings bordering Pak Hoi Street, Temple Street, Ning Po Street and Reclamation Street, and testing there must be performed even in the absence of any new infections.

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Also on Tuesday, the system requiring testing after just one case was extended to an area of Sham Shui Po bordered by Yen Chow Street, Tai Po Road, Maple Street and Lai Chi Kok Road.

Officials reported on Wednesday that environmental samples had been collected from two households at Hiu Fung House in Fung Wah Estate, Chai Wan, where residents from all number 13 flats were evacuated after cases were recorded across different floors. So far, more than 60 people from the block have been quarantined.

Three of the 11 samples taken from exhaust fans, floors, sewers and the surface of toilet seats returned positive, albeit with low viral loads, the authorities found.

Among the dozens of people newly testing preliminary-positive across Hong Kong was a nurse who worked for the past three months on a Covid-19 isolation ward in North District Hospital. She had met up with five colleagues, who were identified as close contacts and sent to quarantine.

District councillor Kalvin Ho Kai-ming questioned the government’s decision to step up testing in Sham Shui Po, noting neighbourhoods in Kowloon City and Yuen Long had recorded similarly high numbers of cases without having to undergo tighter screening processes.

“If officials are saying that areas with many old tenement buildings should be included for more stringent screening, plenty of other areas also have the same problem. Why did the government target this neighbourhood in particular? They should further explain,” Ho told a radio programme on Wednesday.

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But a government source said there were signs of an outbreak in Sham Shui Po area judging from sewage samples that were tested, adding numerous buildings in the area had subdivided flats and some were mismanaged.

Professor David Hui Shu-cheong, a Chinese University respiratory medicine expert and government adviser on the pandemic, said it was necessary to expand screening in the neighbourhood to step up infection control.

“Expanding screening in the Sham Shui Po area does not mean the government is not planning similar requirements for other districts,” Hui told the same radio programme.

University of Hong Kong microbiologist Ho Pak-leung, meanwhile, was among those who rejected the notion that the recent surge in Covid-19 cases was connected solely to the number of people being tested.

“The situation is not improving yet,” Ho said. “If virus transmission is not active in the community, a spike in cases may not happen, even if there is a higher number of people being tested.”

Additional reporting by Elizabeth Cheung