Ventilation systems are unlikely to be the transmission route of eight coronavirus cases detected at an apartment complex in Guro-gu, western Seoul, health authorities said Thursday, while saying it was looking into all possibilities.

Eight people residing in the same building have been confirmed to have contracted the virus since the first case was detected there on Aug. 23. They are from five households living both above and below the first patient’s apartment. Twenty more cases linked to the apartment cluster were reported as of Thursday.

Kwon Jun-wook, deputy director of the Central Disease Control Headquarters said the chance that the virus was transmitted through vents was slim.

“We haven’t completed epidemiological investigation yet but a patient who showed more developed symptoms than others was living in upper floors,” he said.

On Thursday, experts have begun an investigation into the apartment complex to identify the COVID-19 transmission route, according to the Seoul Metropolitan Government.

According to Park Yoo-mi, the city’s director of public health, environmental specimen tests on 11 sites of the apartment including vents at kitchens and bathrooms, sewers and elevators are underway to discover paths of transmission of the coronavirus.

“The first field investigation is underway from 10 a.m. today to confirm the path of infection with epidemiological experts, construction experts, equipment experts, and the Korea Centers for Disease Control and Prevention, and the Guro-gu district office,” she said during an online press briefing.

Park said there have been no reports of the virus spreading through vents in the country but the city will consider all possibilities while investigating the transmission route.

“A lot of evidence is needed to conclude that it was spread through a vent. It’s just one of several possible routes. Presuming the vents as the cause may be misleading,” a KCDC official said.

The apartment complex has 268 households and 500 residents.

Of some 436 residents who underwent testing at a makeshift testing center set up in front of the apartment by the city government, eight people, including the first case, tested positive.

Twenty more cases were detected at a livestock product processing company in Geumcheon-gu, where one of the apartment residents who tested positive works.

The city government classifies the cases involving the livestock product company as Guro-gu apartment-related cases but the chronological order of virus occurrence between the two sites has not been confirmed.

By Park Han-na (hnpark@heraldcorp.com)