Yuen’s comments came after he inspected a residential block in North Point on Wednesday, suspecting vertical transmission of the virus there.
Fourteen households who live in Flat B in phase three of Maple Gardens, on Kin Wah Street, have to be evacuated and sent into quarantine.
After the infection of two people who live in 6B earlier, a man, 32, who lives three floors above in 9B, has tested preliminary positive.
“Because there’s renovation work on the outer walls of the building, the resident in 9B usually closes all the windows,” Yuen said.
“In addition, he turns on the ventilation fan in the toilet, so there’s negative pressure, it’s easier to draw air in the toilet pipes back indoors.”
Yuen said the U-trap of the bathroom pipe in 9B was dry, which might have caused the spread of the virus.
The University of Hong Kong microbiologist added he didn’t see evidence of hidden chains of transmission in North Point.
Yuen was also asked if people should wear the N-95 mask as advised by the authorities in the United States. He said that’s not necessary as Hong Kong has far fewer coronavirus cases compared with the US.
He said if people want to be extra cautious, they can wear a surgical mask and a cloth mask at the same time.
Yuen also repeated calls for people to get vaccinated.