Residents who lived in unit 09 on all floors of Oi Fai House in Yau Oi Estate were ordered to leave and sent into quarantine.

This came as two more residents of the building, a 66-year-old man and a six-year-old boy, became the latest preliminary-positive cases health authorities are looking into.

Three of the cases involved residents of the flat on the 21st and 23rd floor, while the fourth infection lives in a different unit on the 22nd floor.

After inspecting the building, Professor Yuen Kwok-yung from the University of Hong Kong said one of the flats had two exhaust fans in the kitchen, and it appeared that the kitchen was modified and directly linked to the toilet.

Yuen, also a government adviser on the pandemic response, suspected the exhaust fans might have sucked the virus in the pipes after creating negative pressure in the rooms.

“According to epidemiological investigations, [resident of] unit 2309 was first confirmed with the virus. It’s likely that the virus travelled downwards to unit 2209, and then unit 2109,” said Yuen on Friday evening.

He added the patient who lived in unit 2231 might have picked up the virus via a horizontal transmission from unit 2209.

“We cannot tell if the transmission was going up or down at the moment, but, likely, the virus spread downwards. In view of this, we are suggesting residents [of the same unit] to be evacuated tonight.”