“I feel for these people. It’s tough,” Associate Professor Russo said.
“But the evidence at this stage is an 18-fold increase of transmission risk indoors, compared to outdoor environments.”
“Smaller, enclosed, crowded, indoor environments … are where we’re seeing the highest risk of transmission.”
Restaurant & Catering Australia chief executive Wes Lambert has submitted a blueprint to the state government to reopen dining.
“There is no science in Australia, the country we live in, that shows dining indoors or outdoors is any safer,” he said.
“The anecdotal evidence … is an infinitesimal number of the cases in Australia can be traced to hospitality businesses.”
New York, which has managed to hold new cases low for months, has banned indoor dining. Meanwhile San Diego traced nearly a third of its outbreaks to restaurants and bars; Louisiana traced nearly a quarter.
A single hospitality employee who worked at multiple venues was linked to one of Melbourne’s first major clusters; a single person infected at least 34 others after visiting Sydney’s Crossroads Hotel.
A new US government study of 314 people found people with COVID-19 were twice as likely to have dined at a restaurant than those without the virus, although this does not mean they caught the virus there.
Earlier this month, Victoria’s Chief Health Officer Brett Sutton told media the risk of COVID-19 transmission was about 20 times higher inside than outside.
“If you are indoors for a protracted period of time that is exactly when transmission occurs. People need to be aware of that,” he said.
It is not clear, scientists said, why the virus spread more effectively indoors.
It is likely people are closer to each other. Airborne spread – where tiny particles of virus hang for hours in stuffy indoor air – may also play a role. Air dried out by air-conditioning may help.
La Trobe University epidemiologist Associate Professor Hassan Vally said: “We know indoors is worse than outdoors.”
About 80 per cent of all coronavirus cases are spread by “super-spreaders”, and these infections occur almost exclusively indoors.
University of Newcastle clinical epidemiologist Professor Craig Dalton said eating outdoors was “extremely low risk” when compared with indoor dining.
The vast majority of super-spreading occurs indoors in spaces where there are crowds of people and poor ventilation, he said.
“There have been some clusters in cafes and restaurants, but I don’t think many of them had fully implemented appropriate policies yet, like increasing ventilation, which could greatly reduce the risk,” the public health physician said.
A database maintained by Koen Swinkels records that, of 1511 recorded super-spreading events, 1438 happened indoors. There are only three known outdoor super-spreading events.
Liam is The Age and Sydney Morning Herald’s science reporter
Melissa Cunningham is The Age’s health reporter.