Melbourne’s mask rule: is there evidence Covid spreads outdoors and how does it occur?

·6-min read
<span>Photograph: Sandra Sanders/Reuters</span>
Photograph: Sandra Sanders/Reuters

Hours before Victorians came out of lockdown this week, the acting premier, James Merlino, announced a rule scheduled to end would instead remain in place.

Masks, Merlino said on Thursday, would still need to be worn outdoors.

Health authorities backtracked on the plan to lift the mandatory requirement after four members of the same family tested positive to Covid. It led to health authorities being pressed on why outdoor masks were required when transmission throughout Victoria’s most recent outbreak had occurred indoors – whether in stores, family homes, or workplaces.

Victoria’s Covid-19 testing commander, Jeroen Weimar, confirmed “we have no evidence that we’ve seen yet, in this particular outbreak or the most recent ones we’ve dealt with, of outdoor transmission”.

Related: Victoria Covid update: lockdown lifts but strict health orders remain as no new local cases recorded

Weimar also said: “The clear advice from the public health team … is we need to continue with wearing masks outdoors. It is inconvenient but an easy thing to do.”

So why is the restriction necessary and what does the latest evidence say about outdoor Covid-19 spread?

Outdoor transmission can and does occur

Though seemingly not a factor in Victoria’s situation, outdoor transmission does occur and it is something Victorian authorities clearly want to avoid after managing to get down to a day of zero new cases of community transmission on Friday.

Perhaps the most notable example is the September White House event that occurred outdoors in the Rose Garden, and which saw a dozen people test positive within days, including the then-US president, Donald Trump. However, that crowd was at 200, they shook hands and hugged, and many did not wear masks. Restrictions still in place in Victoria mean people can only gather outdoors in public places in groups of 10, and even then must take precautions.

Prof Tony Blakely, an epidemiologist and public health medicine specialist, said just because no cases of outdoor transmission had been recorded in Victoria, it did not mean it had not occurred during outbreaks. It would be really hard to detect outdoor transmission, even if it did occur, he said.

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“You don’t have a QR code you generate as you walk down the street, and contact tracing is easier with indoor buildings because it’s much easier to remember specifics about going into a venue, what you did while you were there … when you log in,” Blakely said.

“It’s harder to recall or know who you passed, and where exactly you went, while you were walking through a park or down a street.

“Although we can’t at this stage say how much outdoor transmission happens and we may never know, the annoyance of wearing a mask may be worth it if it prevents even a couple of cases. It’s a measure health authorities can put in place that doesn’t impact your ability to explore and travel within your 25km radius. It’s just a bit inconvenient.”

Is there any evidence for outdoor transmission risk, and how it occurs?

Risk can be difficult to determine because of the number of variables, such as the public health measures in place, population density and crowding, the rate of Covid-19 in the community, the infectiousness of the variant, the wind, and other factors difficult to tease out and apart. It also depends on how well health authorities record new cases and contact trace.

A systematic review of peer-reviewed papers led by the University of California and published in the Journal of Infectious Diseases in February found even when outdoor transmission does occur, little is known about the pathway of transmission.

The review identified five studies that reported outdoor Covid-19 infections and found a low proportion – less than 10% – occurred outdoors, and the odds of indoor transmission was almost 19 times higher.

Related: ‘Extremely rare’: Australia records second death ‘likely linked’ to AstraZeneca vaccine blood clots

“In general, factors such as duration and frequency of personal contact, lack of personal protective equipment, and occasional indoor gathering during a largely outdoor experience were associated with outdoor reports of infection,” the review found.

Another study published in February in the Euro-Mediterranean Journal for Environmental Integration and led by researchers with Italy’s University of Salerno said there are some outdoor transmission pathways that had perhaps been underestimated and needed further research.

“Outdoor risk sources such as aerosolised particles emitted during wastewater treatment and particulate matter, both of which may act as virus carriers, should be examined,” the paper said. “These aspects of coronavirus propagation need to be accounted for when devising actions to limit not only the current pandemic but also future outbreaks.”

So do Victoria’s outdoor mask rules make sense?

It does depend on which expert you speak to. Some say when there is even a small risk, any relatively easy precautionary measure you can take to prevent spread is worthwhile, and mask-wearing is pretty accessible and straightforward.

“However, I can see no issue with someone removing their mask while walking across a park with no one else around, and just putting it on if they come across people, like at a pedestrian crossing,” Blakely said.

Others believe given how strong Victoria’s public health measures are and how little virus remains in the community, there is a risk the public will begin to fatigue with excessive health measures especially in Victoria where people have spent months of the past year in lockdown. There is also the added threat of fines for not complying with the rules.

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On Friday, Victoria’s chief health officer, Brett Sutton, said: “People move in and out of indoor and outdoor areas all the time, and it is not always easy to judge how close others are.” For this reason, and until more was known about the most recent four cases in the same family, masks made sense, he said. Sutton did acknowledge that outdoor transmission was “a substantially smaller risk”.

“That is why we encourage a whole lot of activities to take place outdoors, and for individuals to meet outdoors, but transmission still occurs,” he said. “Obviously we do not want to impose things that are excessive, but it is always under review, because the risk remains until we have run all of this down.”

Another public health expert who could not be named due to his work with the government told Guardian Australia the outdoor mask rule was “complete BS”.

Prof Catherine Bennett, the chair of epidemiology at Deakin University, said while there may be an argument for masks in areas of community transmission such as Epping, Craigieburn and Port Melbourne, “the risk outdoors beyond those areas is so low now that masks are unlikely to add anything outdoors”.

“The argument that wearing them all the time makes people more likely to also wear them indoors or when they can’t adequately space outdoors is unproven and it would be good to see this evaluated so we know for next time,” she said.

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