It could take just a brief encounter in an MTR station for the Omicron variant to transmit from one person to another, Hong Kong health officials said on Thursday.

Contact tracing of a previously unlinked, positive case found possible transmission pathways, revealing new details about the infection of a 26-year-old kindergarten teacher who lived in Mei Foo Sun Chuen.

She was on her way to work when she encountered two infected people in an MTR station tunnel.

CCTV footage and her Octopus card transaction history showed that she went through a tunnel at a Mei Foo MTR exit at almost the same time – just nine seconds apart – as the two people last Friday. All three wore masks, and the teacher did not have any direct contact with the pair.

Government pandemic adviser Professor David Hui.

Government pandemic adviser Professor David Hui Shu-cheong explained that such transmission is possible, given that the transmissibility of the Omicron variant is four to eight times higher than the Delta variant.

“If the index [patient] was near the kindergarten teacher, the wind direction could carry the droplets [containing virus] to the teacher,” he said. “Although a surgical mask could protect you from large droplets, it could still reach your eyes or leak in when you rub your eyes and nose.”

He added that short-range airborne transmission could also occur if they stayed at a close distance, through gaps at the sides of masks. He suggested residents wear glasses, plain spectacles or face shields on top of face masks for extra protection.

Health authorities have thus linked the kindergarten teacher to a cluster sparked by a woman returning from Pakistan who tested positive for the Omicron variant days after her three-week hotel quarantine had ended. The genetic sequence result of the kindergarten teacher and woman are identical.

As of Thursday, the cluster has 24 people.

People queue for tests in Mei Foo.

Dr Chuang Shuk-kwan, head of the Centre for Health Protection’s communicable disease branch said: “This is the only possible common path we could find at this moment. Of course, if there are other instances such as [eating in the same restaurant] … that may explain the infection more.”

In early January, a distant transmission involving a 42-year-old surveyor turned out to be a false alarm.

Health authorities initially identified he might be getting infected when he passed by Victoria Park where infected people were dancing 20 metres apart, but it was later found that he had breakfast with an infected diner at a restaurant.

As of Thursday, the city’s official tally stands at 13,096, with 213 related deaths. Officials said they were unable to determine the source of four of the latest cases, taking the total number of untraceable infections in the current outbreak to six.