The Introduction of a Mandatory Mask Policy Was Associated with Significantly Reduced COVID-19 Cases in a Major Metropolitan City

Background: Whilst evidence of face masks’ effectiveness in reducing COVID-19 cases is increasing, the impact of mandatory use across a large population has been difficult to assess. Introduction of mandatory mask use on July 22, 2020 during a resurgence of COVID-19 in Melbourne, Australia created a natural experiment to assess the impact of the policy on the epidemic growth rate as its introduction occurred in the absence of other changes to restrictions. &nbsp;<br><br>Methods: Exponential epidemic growth or decay rates in daily COVID-19 diagnoses were estimated using a non-weighted linear regression of the natural logarithm of the daily cases against time, using a linear spline model with one knot (lspline package in R v 3.6.3). The model’s two linear segments straddle the hinge day, determined from the model fit, on which the mask policy began to take effect. We used two forms of data to assess change in mask usage: population-based survey data and images of people wearing masks in public places obtained from a major media outlet. Potential confounding factors (including daily COVID-19 tests, number of COVID-19 cases among population subsets affected differentially by the mask policy – e.g. healthcare workers) were examined for their impact on the results.<br><br>Findings: Introduction of a mandatory mask policy was associated with an estimated 23% reduction in the effective reproduction ratio, from 1.18 to 0.91. The hinge day in the linear spline model occurred eight days after the policy was introduced on July 23, 2020. Daily cases grew exponentially in the first log-linear segment (k = +0.042, s.e. = 0.007), and decayed exponentially in the second (k = -0.023, s.e. = 0.017). Analysis of images of people in public spaces showed mask usage rose from approximately 43% to 97%. Analysis of survey data found that on the third day before policy introduction, 44% of participants reported “often” or “always” wearing a mask; on the fourth day after, 100% reported “always” doing so. No potentially confounding factors were associated with the observed change in growth rates.<br><br>Interpretation: The mandatory mask use policy substantially increased public use of masks and was associated with a significant decline in COVID-19 transmission after introduction of the policy. The hinge date (where the shift in epidemic growth was observed) occurred eight days after policy introduction, consistent with the incubation time of COVID-19 and the time needed to test and report new cases. Although studies emphasise&nbsp;the importance of masks in a clinical setting, to our knowledge no previous peer reviewed publications measured the role of masks for controlling epidemics in the broader community.<br><br>Funding Statement: This work was funded from Burnet Institute resources.<br><br>Declaration of Interests: BC serves on the National COVID-19 Health Research Advisory Committee that provides guidance to the Federal Government’s COVID-19 response. NS, MH, A Saeri, EG and LS provide guidance to the Victorian Government’s COVID-19 response. MH and NS receive funding from the Victorian Government for the Optimise Study and modelling work. A Saeri and EG receive funding from the Victorian Government for the SCRUB project. CRM serves on the National COVID-19 Health Research Advisory Committee that provides guidance to the Federal Government’s COVID-19 response. She currently receives funding from the National Health and Medical Research Council and Medical Research Futures Fund, Office of Health Protection, Department of Health, Sanofi and from the Defence Intelligence Organisation. All authors have nothing to declare.