Another resident from a third flat tested preliminary-positive for the virus.

Dr Chuang Shuk-kwan, head of the communicable disease branch of the Centre for Health Protection (CHP), said all three flats were numbered 9.

“Therefore, we are worried that there might be vertical transmission,” Chuang said, adding that officers from the Environmental Protection Department and the Housing Department also joined the site inspection.

The three flats in question were located on the 14th, 16th and 26th floors.

After the inspection, Yuen said experts could not determine if the source of infection came from flat 1409, where the first case was reported in the building.

“We cannot rule out that the virus could have first spread from residents living below flat 1409 or above flat 2609, because there were only one to two days between the onset of symptoms and the return of positive test results,” he said.

Yuen noted that vent pipes in some flats had been altered or cut off and sealed, and the condition of some pipes on the external walls of the building was not ideal.

“We saw some pipes outside the building that had grass growing out of them, which means there may be some leakage,” he said.

Meanwhile, Dr Albert Au Ka-wing from the CHP said the infections in the housing block might have come from the same source.

“The onset of symptoms among the six patients was very close, between January 10 and 12, so they could have had the same source of infection,” he said.

Vincent Ho Kui-yip, a former president of the Hong Kong Institute of Surveyors, believed that the problem came from anti-siphonage pipes that were cut off in some flats.

He said a virus and other pathogens could enter a flat more easily after such pipes, meant to provide a condition where water can be sealed in toilet bowls, were altered.

“If there is flushing of the toilet in a person’s flat or from the one above, the sealed water could be sucked out. This could be a route of virus transmission,” Ho said.

He added that if the pipe was not sealed properly after being cut, foul gas from other homes could also enter a flat.

Yan Shek House was one of three buildings issued with mandatory testing orders on Thursday, along with Block 1 of The Spectra in Yuen Long and 163-165 Shanghai Street.

The findings were revealed as the last remaining residents of four ageing tenement blocks in Jordan were evacuated in the morning by officials scrambling to contain a growing cluster at Reclamation Street.

Yau Tsim Mong district has reported 81 local cases in the past two weeks, including 27 from the four tenement blocks at 20, 22, 24 and 26 Reclamation Street.

Chuang said subdivided flats and ethnic minority communities were commonly found in the area and the surge in cases was likely due to recent outbreaks at two construction sites.

“Many workers at the construction sites of the central tunnel of the Central Kowloon Route and the Tseung Kwan O-Lam Tin Tunnel live there. Most of them are South Asians, such as Nepalis, and there are quite a lot of members in each family,” Chuang said. “There might be transmissions within the families or with friends in the neighbourhood.”

Chuang also defended the overnight evacuation of residents, a U-turn from her stance over the last two days, saying the decision was reached late on Wednesday evening after taking into account residents’ views and the pipe conditions there.

Dr Leung Chi-chiu, a respiratory medicine specialist, said the quarantine decision came late. “As there are subdivided units with many risk factors, even if there is only one infected household, it would still be reasonable to put residents [living above or below] under mandatory quarantine.”

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Phyllis Cheung Fung-mei, executive director of Hong Kong Unison, a group advocating rights for ethnic minority groups in the city, said most residents she had been in touch with were caught off guard by the late-night evacuation. Cheung said authorities should have been more culturally sensitive, such as providing interpretation services when going door to door in the neighbourhood with a large Nepalese diaspora.

The government will offer a one-off free virus test for all construction workers in Hong Kong at the city’s 19 community testing centres and step up education about epidemic control in the district.

The free testing service, which requires online booking in advance, begins on Friday and will last till the end of the month. Mandatory testing for residents of the four Reclamation Street buildings as well as recent visitors is ongoing.

As of 9am, at least 79 residents of the four blocks, home to numerous subdivided flats, had been moved into quarantine. Police officers would also be sent to clear the buildings again as some residents went back home after work. One resident at 24 Reclamation Street was confirmed infected on Thursday, and another tested initially positive.

A government source said an official inspection of the properties’ drainage systems would be made.

Households had earlier pleaded for help from the government to move to a temporary safe space, complaining of poor plumbing and ventilation.

Three of Thursday’s confirmed cases were imported and the rest local infections, including 13 from unknown sources. More than 30 people also tested preliminary-positive for the virus.

The city has so far recorded 9,414 confirmed infections with 161 related deaths.

Separately, Undersecretary for Food and Health Dr Chui Tak-yi told the press the purchase of other Covid-19 vaccines such as a type developed by Sinopharm was “not the most pressing task”. His comments followed calls by a lawmaker in the pro-establishment camp to obtain the vaccine, which was being rolled out across mainland China.

Additional reporting by Zoe Low