With Australia focused on controlling multiple COVID-19 outbreaks in 2021, cast your mind back to Melbourne mid-2020. In 8 weeks between 14 June & 10 August, Victoria recorded 13,078 cases of COVID-19 with daily cases peaking at 686 on 5th August - almost all in Melbourne. 1/11 
In response: 1) ‘Stage 3’ restrictions - social venues shut, public gatherings barred, stay at home orders – were introduced in select areas on 1 July & across Melbourne on 8 July; 2) mandatory #masks introduced on 22 July; & 3) restrictions strengthened in early August. 2/11
What evolved in Melbourne in 2020 represents one of the most successful public health responses to COVID-19 globally. By October daily case numbers were mostly in single figures & in November no new cases were detected. So what was the turning point for Melbourne? 3/11
What evolved in Melbourne in 2020 represents one of the most successful public health responses to COVID-19 globally. By October daily case numbers were mostly in single figures & in November no new cases were detected. So what was the turning point for Melbourne? 3/11
Distinct response phases well separated in time provided an ideal natural experiment to assess their relative impact. Our analyses of growth rates in daily COVID-19 diagnoses found a highly significant decrease in cases coincided with introducing a mandatory #mask policy. 4/11
While Stage 3 restrictions had some impact in slowing the growth rate in daily diagnoses, it wasn’t until a mandatory #masks policy was introduced - with very high compliance levels evident in observational and survey data - that the daily case rates went into decline. 5/11
After Stage 3 but before #masks daily case rate still grew exponentially, doubling every 16.5 days. 8 days after #masks (lag accounts for SARS-CoV-2 generation interval & infection-testing-reporting time), daily case rate went into exponential decline, halving every 30 days. 6/11
After Stage 3 but before #masks daily case rate still grew exponentially, doubling every 16.5 days. 8 days after #masks (lag accounts for SARS-CoV-2 generation interval & infection-testing-reporting time), daily case rate went into exponential decline, halving every 30 days. 6/11
A range of alternate explanations were examined – change in # daily tests, mobility data, ambient temperature, cases in health care workers/regional areas, cases assigned to clusters – but none could explain the change seen after the #mask policy was introduced. 7/11
A range of alternate explanations were examined – change in # daily tests, mobility data, ambient temperature, cases in health care workers/regional areas, cases assigned to clusters – but none could explain the change seen after the #mask policy was introduced. 7/11
Our analyses, alongside the now uncontested position that SARS-CoV-2 is mostly transmitted airborne via aerosols, underscores that #masks play a crucial role in helping control COVID-19 spread, in combination with high vaccination coverage and other preventive measures. 8/11
Melbourne mandated #masks anywhere outside of people’s homes. While COVID-19 transmission risk outdoors is contested, risk is not zero. Given the low social & economic burden of mask wearing, judicious use of #masks across all settings, including outdoors, is warranted. 9/11
As Australia and other countries (including those with ⬆️ vaccination rates) struggle to control highly infectious Delta variant outbreaks where transmissions from fleeting contacts (including outdoors) are occurring, #masks must remain a key component of our response. 10/11
#PLOSONE: The introduction of a mandatory mask policy was associated with significantly reduced COVID-19 case ... dx.plos.org/10.1371/journa… @BurnetInstitute @MargaretHellard @CrabbBrendan @AllanSaul7 @KnowC19_Burnet 11/11
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