Airflow and seating likely factors in Thai restaurant cluster

The Black Rock restaurant “super-spreading” event at the centre of Victoria’s latest coronavirus outbreak was likely fuelled by the venue’s airflow and seating arrangements, despite the restaurant abiding by density rules.

Eleven of the 37 patrons who dined at the Smile Buffalo Thai restaurant on the night of December 21 tested positive to the virus, after visiting between 7pm and 9.30pm.

Eleven patrons and one staff member at the Smile Buffalo Thai restaurant at Black Rock have been infected with the coronavirus.Credit:Penny Stephens

Significantly, the group were not all on the same table, nor even in same area of the restaurant. They sat on four tables, in different areas. A staff member also tested positive.

While the identity of the person or people who were infectious with coronavirus that night is still yet to be discovered, the Victorian government now believe that their level of infectivity, as well as seating arrangements and airflow, likely contributed to the transmission of the disease.

Burnet Institute epidemiologist Professor Mike Toole said the 12 coronavirus cases were a mini super-spreading event, but one that would not have made sense nine months ago when it was thought that coronavirus was mostly spread by large droplets.

He said it seemed the virus had spread via finer particles known as aerosols, meaning social distancing might not always be adequate protection from the disease, particularly in poorly ventilated rooms where people were speaking loudly.

“The traditional 1.5 metres apart isn’t enough in that setting, so masks become absolutely critical," he said.

Professor Jason Monty says air flow can be improved by setting up a fan at a window.

Victorian businesses operators are also advised to avoid interactions in closed spaces and improve ventilation by opening windows and doors as part of their COVID-safe plans.

Professor Jason Monty, the head of mechanical engineering at the University of Melbourne, said some small restaurants didn’t have any ventilation other than doorways or a split-system air-conditioner, which blowed air in one direction around the room.

He said opening windows might be only of limited benefit in these circumstances because there was nothing pushing the air out of the shop, so he recommended setting up a fan at the window to push the air out, or an air purifier.

“We need to put a bit more effort into encouraging people to try to clean air in their restaurants," he said.

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