The term 'fleeting transmission', now synonymous with Covid outbreaks fuelled by the highly-infectious Delta variant, has again made headlines this week.
"We're back in the world of very fleeting transmission," Victoria's testing chief Jeroen Weimar said on Friday describing infections at the MCG as his state entered a snap, five-day lockdown.
On Sunday, an expert in healthcare moved to portray just how devastating "fleeting transmission” of the Delta variant can be and used a daunting analogy to explain it.
Dr Stephen Duckett, director of the health program at Grattan Institute, told ABC Weekend Breakfast what we knew about coronavirus has “dramatically changed over the last 18 months”.
“So last year we assumed that the virus was transmitted through droplet transmission, so the droplets would come out of your mouth and drop to the ground, and this is where we got this 1.5 metre rule,” Dr Duckett said.
“What we now know is that the virus is transmitted through aerosol spread, sort of like cigarette smoke.
"So if you imagine that you are somewhere near a smoker and you can smell their cigarette smoke, it is the same with the virus, that the virus is wandering around you and so obviously you can catch it through that mechanism.
“So certainly masks are much more prevalent - masks compulsory indoors in Victoria, and it should change our behaviour to be just that much more careful when we're near other people to make sure we are protecting ourselves with our masks.”
Examples of 'fleeting transmission'
Mr Weimar went into detail on Friday about the four men infected with the virus at the MCG.
All four contracted coronavirus from another man at the stadium but has little to do with the case.
"They did not know each other, did not sit directly with each other, did not know the index case or his friend," Mr Weimar revealed.
Mr Weimar said transmission most likely occurred when the infected man went to the bar or the toilet.
NSW Premier Gladys Berejiklian last month described a number of cases as “scarily fleeting”. NSW's Chief Health Officer Dr Kerry Chant previously expressed her concern over such transmission occurring in retail settings which had previously not posed a threat.
At the beginning of the state’s latest outbreak in June, she said it was "particularly concerning" to see Bondi Junction Westfield become a transmission site.
"I don't want to enhance [the idea] this virus jumped across a room or anything... but [after viewing CCTV footage] it was inadvertent contact you would have in retail settings that we would not have suspected transmission could happen," she said.
Aerosol expert explains Delta threat
Associate Professor Ben Mullins, an aerosol deposition expert at Curtin University’s School of Public Health, perviously told Yahoo News Australia there is evidence the Delta strain is more prone to aerosol transmission than other variants.
"That coupled with the weather conditions at this time of year means aerosolised viruses will stay longer in the air,” he said.
It also means some sites which were once deemed low risk are no longer deemed that way.
"There's basically a cloud of aerosols in the air that's been exhaled by someone with the Delta variant and then someone else will walk through that and inhale a sufficient viral load to be infected Associate Professor Mullins explained.
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