A housing block in Sai Ying Pun is being partially evacuated after a resident there came down with the coronavirus.

The patient, 25, lives in a flat on the fourth floor of Block B at Kwan Yick Building Phase Three, where three others diagnosed with the virus earlier live in the unit directly below.

He works at a property management company on Portland Street in Yau Ma Tei. It was the only local case reported on Wednesday, with no clear source of infection.

After inspecting the building, infectious disease expert Yuen Kwok-yung said he believes the virus could have spread vertically through the air.

Those living in flat 10 from the 2nd floor or above at Block B will be evacuated, while other residents would undergo compulsory testing. The expert said they should avoid leaving the building.

Yuen said while he didn’t find any issues with the pipes in the block, he noted that flats 9 and 10 share light wells.

He said when residents turn on exhaust fans in toilets and kitchens, it would create negative pressure indoors, pushing air in the light well into the flat.

The expert said since an infected person living on the third floor had a high viral load, it was possible that the respiratory droplets from this person were sucked out of the kitchen or toilet and directed vertically into the flat above.

He said another, less likely, possibility was that the U-shaped water pipes had failed to prevent the spread of the virus.

Kwan Yick Building was the target of an ambush-style lockdown two weeks ago, but the man tested negative at the time.

At a press briefing earlier, Dr Chuang Chuk-kwan of the Centre for Health Protection says authorities are not sure yet how he was infected. She said he first showed symptoms on Monday, about 12 days after the lockdown, and may have been in the incubation period at the time.