“Even though daily cases have dipped slightly compared with before, there is no sustained downward trend, and the decline is slower than what we saw in the city’s third wave,” Chui told a daily Covid-19 briefing.

Although the total number of cases fell to 434 last week, from 614 a week earlier, untraceable infections had held steady, with the proportion rising slightly from 33 to 36 per cent, he said. The threshold for mandatory testing would therefore be lowered from four unrelated cases in a block to two, Chui said.

The pilot sewage monitoring scheme, which found coronavirus traces in consecutive sampling of pipes from Fung Chak House of Choi Wan (II) Estate, prompted officials to order mandatory testing for residents of the building and recent visitors on Monday.

Chui revealed that three residents tested preliminary-positive for the virus. A total of 721 people were screened at a mobile testing station near the block on Monday, and 2,027 on Sunday and Monday at another makeshift site set up earlier in the area.

‘Sustained’ evidence of Covid-19 found in Hong Kong housing block’s sewage

Dr Chuang Shuk-kwan, head of the Centre for Health Protection’s communicable disease branch, said two of the patients were from the same household. One of them developed a fever on December 24 and the other had a cough on Christmas Day.

The third patient, from another family, had a cough on December 28. None of them, though symptomatic, had sought medical attention or a test previously.

“The initial data shows sewage sampling has some reference value for deciding whether to do mandatory testing,” Chui said.

Undersecretary for the Environment Tse Chin-wan said the government’s current thinking was to collect and test more sewage samples from buildings that had completed mandatory testing, so the authorities could check whether any residual carriers remained.

Professor Leo Poon Lit-man, head of the division of laboratory sciences at the University of Hong Kong’s school of public health, told a Tuesday morning radio show that the monitoring system could complement other testing regimes and help policymaking, but admitted the method could be time-consuming, and it was difficult to test every block in the city.

Chuang said the new tightened testing protocol meant Sha Tin’s Wing Sui House, which had seven infections, and Lam Shek House in Ping Shek estate, with three patients, would be put under a mandatory screening order.

While admitting the new threshold could easily be met, meaning residents could be hit with a compulsory testing order at short notice, Chuang said it was better to be safe than sorry when fighting the virus.

Checkpoint uncovers 76 Hongkongers who violated Covid-19 testing order

The health official also revealed that five more infected travellers, who arrived in Hong Kong between December 4 and 20, carried the new, more transmissible variant of the virus, which was first seen in Britain. Together with three other cases reported earlier this month, eight patients were now infected with the variant.

A team of infectious disease experts in Hong Kong had earlier found the new variant was about 75 per cent more transmissible than the previous strain circulating in the city.

Respiratory medicine specialist Dr Leung Chi-chiu said the discovery of eight cases out of thousands of students coming home from Britain was not a huge concern.

“But the government needs to trace where the infected have been, who they were in contact with, and conduct testing on all passengers on the planes the infected flew in on,” Leung said.

Two travellers who arrived from South Africa on December 16 were also confirmed as infected on Tuesday. The patients, who had contact with each other, did not show any symptoms.

Whether the two had another new variant strain seen in South Africa had yet to be confirmed.

Among other new infections were a nurse at United Christian Hospital, taking the cluster there to 20 cases, and an Indonesian maid who was only confirmed after she finished quarantine.

Hong Kong logs 53 Covid-19 cases as police hunt for woman who fled quarantine

An elderly Covid-19 patient and a 58-year-old man, both with chronic illnesses, became the latest fatalities linked to the health crisis. The elderly woman was the second death connected to the United Christian cluster.

The city’s overall infection tally stood at 8,724, with 143 related deaths. More than 40 people tested preliminary-positive for the virus, pending confirmation.

But Filipino workers returning from these places will be allowed to enter the country. The Philippine government expected up to 100,000 more repatriates would return, and said the new strain had so far not been detected there.

Additional reporting by Zoe Low