Some 100 residents of flats numbered 10 and 12 in Luk Chuen House of Sha Tin's Lek Yuen Estate were evacuated last night, as investigators started probing whether its sewage system could have spread Covid-19 to at least seven residents.

This came as a 72-year-old man from unit 1112 tested positive for coronavirus yesterday. Along with five new imported cases from Bangladesh, the SAR's total tally reached 1,100 yesterday, including four deaths.

The Centre for Health Protection said the evacuation was a precautionary measure as four cases - including a suspected 34-year-old female "superspreader" and her 56-year-old husband in unit 812 - live in unit number 12 of the public housing building.

Despite the 72-year-old's negative sample on Monday, he developed a low fever on Wednesday and was admitted to Prince of Wales Hospital yesterday.

The center's head of communicable disease branch, Chuang Shuk-kwan, said the man mostly stays at home but went out on Saturday and met with a friend outdoor, with both wearing masks.

The cluster emerged on Sunday when the suspected superspreader and her husband were diagnosed, prompting the center to test all 1,200 residents in the 15-story building.

After four more residents - two from other units on the eighth floor, one from 710 and one from 1012 - tested positive, authorities arrived at the block overnight and inspected the pipes and environment but did not find any issues.

Environment undersecretary Tse Chin-wan said authorities did not see the need to evacuate residents on Monday as not all patients shared the same pipe system.

"But now we've found another patient living in unit 12 [so] we cannot rule out the sewage system," he said.

On the testing of other residents, Chuang said 1,200 people have returned their samples but the center is still looking for six households.

"Police went up to their units twice but we still cannot reach them. We don't know if they have moved but we've issued quarantine orders to ask them to contact us," she said.

Residents will be tested again 12 to 13 days later, she added.

Residents of units 10 and 12, which make up 22 units in total, have been sent to Chun Yeung Estate in Fo Tan for a 14-day quarantine.

Meanwhile, five men, 15 to 53, who returned from Bangladesh on Cathay Dragon flight KA734 via Malaysia yesterday, tested positive at the Lei Yue Mun Holiday Camps quarantine center and have been sent to hospital.

100 evacuated amid pipe fears on cluster