It Puts the Mask on Again or

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Open Access

Peer-reviewed

Inquiry Article

The introduction of a mandatory mask policy was associated with significantly reduced COVID-19 cases in a major metropolitan city

• Allan Saul ,
• Tim Spelman,
• Mark Stoove,
• Alisa Pedrana,
• Alexander Saeri,
• Emily Grundy,
• Liam Smith,
• Michael Toole,
• Chandini Raina McIntyre,
• Brendan S. Crabb,
• Margaret Hellard

The introduction of a mandatory mask policy was associated with significantly reduced COVID-xix cases in a major metropolitan city

• Nick Scott,
• Allan Saul,
• Tim Spelman,
• Mark Stoove,
• Alisa Pedrana,
• Alexander Saeri,
• Emily Grundy,
• Liam Smith,
• Michael Toole,
• Chandini Raina McIntyre

ten

• Published: July 21, 2021
• https://doi.org/10.1371/journal.pone.0253510

Figures

Abstract

Background

Whilst show of use of face masks in reducing COVID-nineteen cases is increasing, the touch of mandatory use beyond a large population has been difficult to assess. Introduction of mandatory mask utilize on July 22, 2020 during a resurgence of COVID-19 in Melbourne, Australia created a situation that facilitated an assessment of the affect of the policy on the epidemic growth rate as its introduction occurred in the absenteeism of other changes to restrictions.

Methods and findings

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 2 linear segments pin around the swivel 24-hour interval, on which the mask policy began to take outcome, 8 days following the introduction of the policy. We used ii forms of data to assess modify in mask usage: images of people wearing masks in public places obtained from a major media outlet and population-based survey information. 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 bear upon on the results. Daily cases fitted an exponential growth in the first log-linear segment (k = +0.042, s.eastward. = 0.007), and fitted an exponential decay in the second (k = -0.023, southward.due east. = 0.017) log-linear segment. Over a range of reported serial intervals for SARS-CoV-ii infection, these growth rates correspond to a 22–33% reduction in an effective reproduction ratio before and after mandatory mask utilise. Assay of images of people in public spaces showed mask usage rose from approximately 43% to 97%. Analysis of survey information found that on the tertiary day earlier policy introduction, 44% of participants reported "often" or "ever" wearing a mask; on the fourth twenty-four hours after, 100% reported "e'er" doing and so. No potentially misreckoning factors were associated with the observed alter in growth rates.

Conclusions

The mandatory mask utilise policy substantially increased public use of masks and was associated with a significant reject in new COVID-19 cases later on introduction of the policy. This study strongly supports the utilise of masks for decision-making epidemics in the broader community.

Citation: Scott Northward, Saul A, Spelman T, Stoove M, Pedrana A, Saeri A, et al. (2021) The introduction of a mandatory mask policy was associated with significantly reduced COVID-19 cases in a major metropolitan urban center. PLoS ONE xvi(7): e0253510. https://doi.org/10.1371/journal.pone.0253510

Editor: Thomas A. Smith, Swiss Tropical & Public Wellness Plant, SWITZERLAND

Received: February 10, 2021; Accustomed: June 5, 2021; Published: July 21, 2021

Copyright: © 2021 Scott et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in whatever medium, provided the original author and source are credited.

Data Availability: With one exception, all data are independent in the paper, or available publicly from the websites referenced in the newspaper. The exception is the master photographs used for counting mask usage. These photographs are the copyright of the Historic period Newspaper and can be obtained from photographic archive of The Age (https://www.theage.com.au/) with the contact details listed on their website. The meta information associated with each photograph is listed in the Supplementary file "PhotoMetadata.xlsx".

Funding: The authors received no specific funding for this piece of work.

Competing interests: BC serves on the National COVID-19 Health Research Advisory Committee that provides guidance to the Federal Government's COVID-nineteen 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 Authorities for the Optimise Study and COVID-19 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 Regime's COVID-19 response. She currently receives funding from the National Health and Medical Research Council and Medical Research Futures Fund, Role of Health Protection, Department of Wellness, Sanofi and from the Defense force Intelligence Organisation. This does not change our adherence to PLOS ONE policies on sharing information and materials.

Introduction

Australia, like many countries, experienced a rise in coronavirus affliction (COVID-19) cases in early on 2020, peaking on 28^th March 2020 so declining in Apr after federal and state governments introduced strict community controls, travel bans and quarantining of international arrivals [1]. Throughout late June and July (i.e. the start of the Southern wintertime) at that place was a resurgence of COVID-19 cases confined to Commonwealth of australia's 2nd most populous country, Victoria, with xiii,078 cases detected between 14^th June and x^th August and new daily case numbers peaking at 686 on five^thursday August. Well-nigh all cases (95%) were in the country majuscule, Melbourne, a urban center of 4.93 million people.

The resurgence of COVID-xix in Victoria led to multiple control measures being reintroduced in urban Melbourne, and by the end of September, daily numbers had declined to fewer than 20 and are continuing to decline [2, three]. The beginning major restrictions to be introduced were defined in Victoria as "Stage 3 "restrictions and consisted of:

• closure of pubs, bars, entertainment venues, places of worship, restricting restaurants and cafes to takeaway simply, and limiting public gatherings to ii people; and
• requiring people in the restricted area to stay home, with but four reasons to get out dwelling house: shopping for essentials, caregiving, practice, and work or study that cannot be done from home).

These restrictions were introduced at 11:59pm on 1^st July 2020 in 10 postcodes (out of approximately 550 in Melbourne) that had 70% of the Victorian diagnosed COVID-19 cases at the time. The area covered was expanded on 4^th July to include two boosted postcodes and complete quarantine/isolation of nine public housing tower blocks where residents were considered to be at high risk; residents were required to stay within their homes nether the 'detention' directives, until all residents in each belfry had been tested for presence of SARS-CoV-2 and infected people put into isolation. Depending on the belfry, this took up to one calendar week.

On viii^thursday July, Stage 3 restrictions were extended to include all of metropolitan Melbourne and the bordering Mitchell Shire (hereafter, "Melbourne"), and Victoria's borders with other states/jurisdictions were closed (Table 1). Previous work has shown that the introduction of Phase three restrictions slowed but did non reverse the exponential growth in daily cases; the growth rate fell from 0.140 to 0.037 per twenty-four hour period. This reduction was estimated to take averted 9,000–37,000 new infections throughout July, compared to a scenario of continued trend in growth.

Mandatory employ of masks or other face coverings including surgical masks, cloth masks, face shields, scarves and bandanas (future, masks) in public settings was appear on the 20^th July. The club came into force in Melbourne on 22^nd July with existing Phase 3 restrictions remaining in place. With very few exceptions, the mandatory apply covered all spaces outside the home, everybody 12 and older regardless of social distancing and was strongly enforced with fines of A$200 imposed for not-compliance.

This was followed past an 8pm curfew and 5km movement restrictions introduced on ii^nd August, and Stage four restrictions (divers in Victoria as closure of childcare and non-essential businesses, including retail, and schools delivering online learning only) on 5^thursday August (Tabular array i).

The Earth Health System (WHO), the United States' (Usa) Centre for Disease Command and Prevention (CDC), other public health organisations and governments recommend the use of masks in specific circumstances [four–10]. Notwithstanding, despite these recommendations [4, xi–18] there is express data from community settings supporting mask apply. A systematic review and meta-assay has examined how physical distancing, use of face masks and eye protection affect the spread of COVID-19, SARS, and MERS in both community and healthcare settings [5]. This included 172 observational studies across 16 countries but did not identify any published randomised controlled trials on the effectiveness of masks. The review constitute that mask use (including respirator use) could effect in an 85% chance protection overall, with surgical masks and well-designed material masks giving 67% protection, merely only six studies included data on COVID-19 and only 3 were from non-healthcare settings (and all iii assessed SARS infection). Additional studies accept been published examining the bear on mask utilize (mandatory or consensual) on the incidence of COVID-xix cases [8, 14, nineteen–25], but collectively they illustrate the challenges in estimating the effectiveness of masks in community settings; in all of these studies either the introduction of masks not mandatory (hence the uptake of masks was not an firsthand change making bear upon difficult to measure out), limited information is available most bodily mask uptake, or masks were introduced at the same time every bit other control measures making it difficult to disentangle their impact.

Since this study was undertaken there have been further studies on the touch of masks. A recent CDC update reviewed these studies [26] listing cohort and population-based studies that showed a positive effect by the introduction of or use of masks. One written report on the introduction of masks in Arizona paralleled the situation nosotros describe in this paper: a alter in covid-19 incidence post-obit the land-broad introduction of masks [half-dozen]. although this newspaper does not certificate the speed at which the mask mandate was implemented nor the magnitude of the change in mask usage and the introduction of multiple other measures (closure of bars etc., soon after mask introduction) limiting the generalizability of the conclusions. Notwithstanding, of relevance to our Melbourne study was the ascertainment that the apparent impact on covid-nineteen incidence lagged the introduction of masks past about 12 days.

In July 2020 in Melbourne, the introduction of mandatory masks was strongly enforced and well separated in time from the introduction of other command practices, and with the large and quantified change in mask usage. This provides an of import opportunity to quantitatively compare the pre- and mail-mask intervention epidemic growth and decay rates.

Methods

Analysis of COVID-19 cases x^th July to x^th August inclusive

Daily diagnosed COVID-xix case numbers in Victoria (excluding cases in returned international travellers in hotel quarantine) were derived from the cumulative cases reported in daily updates [two] from the Victorian Department of Wellness and Human Services (DHHS).

The growth or decay of the daily number of new cases was approximated using an exponential model.

This means that the logarithm of the daily cases should increase or decrease linearly with fourth dimension. The growth or decay rates of the epidemic were estimated using a non-weighted linear regression to fit the natural logarithm of the daily cases against fourth dimension, using a linear spline model with one knot [27]. The knot allows for a possible deflection in the line of best fit during the period of observation, dividing the model into ii linear segments that are continued at a swivel day. The assumption of linearity of the ii segments, and hence exponential growth/disuse, was tested past assessing whether studentized residuals were normally distributed and independent of the fourth dimension (i.e., no significant heteroskedasticity). Due to the generation interval for SARS-CoV-two infection and delays in testing and reporting, in that location is an expected delay between the introduction of restrictions and observing them in diagnosis data. The hinge twenty-four hour period was estimated to be eight days following the introduction of masks, based on a previous written report observing an 8 twenty-four hours filibuster from the introduction of Stage three restrictions in Melbourne and a modify in the epidemic growth rate [28], biological plausibility (a mean generation interval estimated at 4–vi days [29, 30], combined with delays in test-seeking and reporting), and assessment of model robustness, with sensitivity analyses used to explore alternate assumptions (S1 File).

This assay is similar to the approach used in a previously published report on the impact of "Stage 3" restrictions in Melbourne, that showed that the growth fitted two exponential segments and also established that the change in worth was consequent with a lag of 8 days. Information technology differs from that written report in using spline fit with a single knot rather than two separate regressions, since this provides a theoretically amend model requiring the segments to share a common incidence on the day that the pre- and post-intervention growth constants diverge.

While this assay estimated the instantaneous growth constant (k) (i.e., the slope of the ln(daily cases) vs fourth dimension regression) for each segment, the difference between these growth constants was used every bit the primary measure to test for an association betwixt masks and epidemic growth/decay rates.

Nosotros accept called the 10^th July as the starting twenty-four hour period for this report, based on previous work observed that the introduction of Phase 3 took eight days to have an observed impact on the growth rates of the daily example data [28]. Since the current study specifically examines the impact of add-on of masks on transmission, to avoid including the bear on of curfew (introduced 2nd August) or Stage 4 restrictions (introduced 5th August) [2] we terminated the study menstruum 8 days after the introduction of the curfew (10th August). Thus, the study period was from ten^th July to x^th August. The menstruation prior to the mandatory introduction of masks on the eleven.59pm on 22 July is referred to as the pre-mask period and the period from the 23^rd July as the mail-mask period.

We calculated the doubling/halving time of the epidemic equally t _two = ln(ii)/k

We also calculated an approximate range of R _eff as the relative change in new cases over 1 serial interval for SARS-CoV-2 infections using the Lotka–Euler equation. Due to uncertainties in the serial interval, we guess a range for R_eff based on two estimates: a normal distribution with hateful 3.96 days and standard deviation iv.75 days based on 468 pairs of infections in Communist china [29], and a gamma distribution with hateful 6.49 days and standard deviation 4.90 days based on 1015 pairs of infections in People's republic of china, Japan and Singapore [xxx]. We have included changes in estimated R _eff only as a secondary estimate to allow comparisons with other studies, but emphasize that the estimates of R _eff are discipline to uncertainty in estimates of the generation interval (approximated as the serial interval) and its distribution.

Photographic observation of mask apply

Nosotros assessed changes in mask-wearing behaviours using images from the digital archive of The Historic period (https://www.theage.com.au/), one of the two major daily Victorian newspapers. To ensure we captured all images (non just those published), the librarian/digital archivist from the paper reviewed consecutive photos in their archive that were taken between July ten and August ii, 2020 (14 days before and 14 days afterward mandatory mask policy introduction). From these, all photos taken in public locations in urban Melbourne, such as streetscapes and shopping centres, and contained clear images of people were extracted and used to calculate the proportion of people wearing masks in public (number of people wearing masks / number of people in the image) across three time periods: period 1, 10–xix July (the period preceding mandatory mask policy announcement); period 2, 20–22 July (period betwixt announcing and commencing mandatory mask wearing); and catamenia iii, 23 July to 2 August (mandatory mask wearing period). We excluded people who were potentially exempt from mask wearing under the relevant directions, such equally children and people drinking or eating while in public. The proportions of individuals wearing masks versus not wearing masks in each period were compared using cross-sectional chi-square tests. The meta data associated with each photograph is listed in the Supplementary file "PhotoMetadata.xlsx". The original photographs tin can be obtained using these metadata from The Age (contact details are on their website).

The Survey of COVID-19 Responses to Understand Behaviour (SCRUB) projection—analysis of mask employ

The Survey of COVID-xix Responses to Understand Behaviour (SCRUB) project includes measures of protective behaviour adherence among Australians during the COVID-19 pandemic; it commenced in March 2020. The survey targets a representative (age, gender, metro/regional) sample of yard Victorian residents and 700 Australian residents exterior Victoria in each round. This study was approved by the Monash Academy Human being Inquiry Ethics Committee (ID 23854). Participants completed and on-line consent class as part of the survey.

An item measuring self-reported mask utilize frequency was included from circular 6 onwards. Hither we include data that spanned the date of mask introduction from round 6 (data collected xx–26 July 2020) for the question "In the past 7 days, how frequently accept you taken the following actions?" and the specific item "Clothing a face mask whenever in public" (response scale: Never, Rarely, Sometimes, Often, Always)., These categories were used to plot the information but were collapsed into a dichotomous outcome to estimate adherence (Never, Rarely, Sometimes versus) Often, Always. Respondent postcode and survey completion date were used to place responses from people living within Melbourne and the daily responses were tabulated.

Possible confounders

Information collected to explore potential confounding effects included one) daily cases from regional Victoria, 2) daily cases in health care workers, 3) daily COVID-19 tests, 4) the proportion of diagnosed cases assigned to a known cluster within 24 hours of testing, and v) mobility indices for Melbourne residents.

Daily cases in regional Victoria were derived from the cumulative cases reported in daily updates. Daily cases in wellness care workers, which were classified as acquired in health care and aged care facilities, were obtained from the Victorian DHHS web site. Daily COVID-19 tests and the proportion of diagnosed cases institute to be close contacts of known clusters (a cluster is two or more than cases previously shown to be close contacts and causeless to have been infected from an initial index case) within 24 hours of testing. This was used as a surrogate to measure changing efficiency in contact tracing over fourth dimension. The data were obtained from the Australian Broadcasting Corporation'due south coronavirus summary [31]. Mobility indices for Melbourne were obtained from CityMapper. The Citymapper Mobility Index is calculated past comparing trips planned in the Citymapper app to an arbitrary reference usage period, to assess whether mobility (relative to the reference period) changed over time during the observation period. The reference menses used was the four weeks between January 6 and February 2, 2020 (i.due east., prior to COVID-19). A day is defined as midnight to midnight UTC, thus in Melbourne overlaps calendar days.

Mask employ in residents of regional Victoria was non mandated and utilize in health intendance and aged care facilities in Melbourne may not accept changed since they were in near universal employ prior to the introduction of the policy. Therefore, we analysed the growth rates in regional Victoria and in health care workers (HCW) on the expectation that these should show a limited change in growth rates in the pre-and post-mask periods. The aged care data were not available for the whole menstruation and were not analysed. We undertook secondary regression analyses using dependent variables of log (total daily cases less regional cases), log (total daily cases less HCW cases) to test if regional cases and HCW cases had a significant touch of the results obtained with the total Victorian cases.

We as well used linear regression of potential confounders (number of tests, log (ratio of total daily cases to number of daily tests), proportion of cases assigned to a cluster and mobility index) every bit a role of fourth dimension to determine if they changed significantly during the observation period.

Finally, nosotros performed a secondary linear spline regression with one knot regression of the log of the Melbourne cases that included daily tests and mobility alphabetize likewise as time as independent variables.

Results

Association of masks with a alter in epidemic growth charge per unit

At that place were 11,714 cases reported in Melbourne between 10^th July and 10^th August inclusive, with daily cases increasing from 143 cases on ten^thursday July 10 to a top of 686 cases on 5^th August, earlier declining to 310 cases on x^thursday Baronial.

The first log-transformed segment of the linear spline model, representing the pre-mask period, was consistent with an exponential growth in daily cases (one thousand = +0.042, standard error [southward.e.] = 0.007; p<0.001). This growth equated to a projected doubling of cases every 16.5 days (95% CI: 12 to 25 days). The 2d log-transformed linear segment, representing the post-mask period, was consistent with an exponential disuse of daily cases (yard = -0.023, s.e. = 0.017; p = 0.190). This disuse equated to a projected halving of cases every thirty days (95% CI: doubling every 65 days to halving every 12 days). The deviation in exponential growth/decay rates between the pre- and post-masks periods was highly statistically meaning (Δk = -0.065, s.eastward. = 0.022; p = 0.006) (Fig one and Table 2).

Fig one. COVID-19 cases detected per 24-hour interval in Victoria.

Observed daily cases (dots) and fitted linear spline model with a hinge day on 31st July, with shading representing upper and lower 95% confidence interval of the fitted curves. Red: assay menstruation for pre-masks; Blue: analysis period for post-masks.

https://doi.org/10.1371/journal.pone.0253510.g001

These growth rates correspond to approximately a 22–33% reduction in R _eff , depending on estimates of the serial interval for SARS-CoV-2. Using a mean serial interval of 3.96 days [29], this corresponds to a 22% refuse in R_eff from 1.xvi in the pre-mask flow to 0.91 in the mail-mask, while using a mean series interval of 6.v days [thirty], this corresponds to a 33% turn down in R_eff from 1.29 to 0.86.

Fig 1 describes a longitudinal comparison of newly detected cases beyond time periods in a unmarried population without explicit reference to a comparator population. This is distinct from a difference-in-difference approach which compares differences in outcomes across multiple time periods between a population of involvement and a comparator or control group. Our analysis has no such comparator group.

On testing our model for goodness-of-fit, the distribution of studentized remainder values of the log transformed cases was indistinguishable from a normal distribution and no show of heteroskedasticity (Breusch-Pagan test) (S1 File). Thus, the data was an excellent fit to an exponential increase in the Stage 3 flow followed by an exponential decrease after masks became mandatory. Sensitivity analysis of the swivel 24-hour interval suggests that viii days following the introduction of masks had a high adjusted coefficient of determination R^two among other choices, and that the estimated change in gradient would be greater for a longer lag time.

Photographic observation of mask utilize

We analysed 44 published photographic images taken between July 10 and August 2, 2020. They included a range of public locations, including streetscapes, markets, train stations and supermarkets, and included 304 different individuals (subsequently excluding those potentially exempt from mask utilise, i.eastward., children estimated to be under 12 and adults while they were eating or drinking). Nineteen photos taken in period ane (prior to mandatory mask policy proclamation) contained 101 different individuals, of whom 43 (43%) were wearing masks. Seven photos taken in period 2 (post-obit mandatory mask policy announcement simply prior to policy introduction) contained 47 unlike individuals, of whom 35 (74%) were wearing masks. Eighteen photos taken in menses 3 (mandatory mask wearing menstruum) contained 156 different individuals, of whom 153 (98%) were wearing masks. The proportion of individuals wearing masks significantly increased from menstruation ane to period ii (χ ⁱⁱ (i) = thirteen.09, p<0.001) and again from period two to catamenia 3 (χ ² (1) = 29.42, p<0.001). Photo metadata are listed in S1 Information.

SCRUB study–self reported mask utilize

Analysis of data from Round vi of the SCRUB study found 44% of participants interviewed in Melbourne on the tertiary day earlier the mask policy was introduced (July 20, 2020) reported they "ofttimes" or "always" wore a mask, and 100% of those interviewed on the fourth mean solar day afterwards (July 26) reported "always" wearing a mask. The survey found that 56% of participants reported "oftentimes" or "always" wearing a mask over 20–22 July, and 71% over 23–26 July (χ ² (i, N = 808) = 8.73, p = 0.002) (Fig ii). This change in mask usage was not observed in SCRUB data collected from participants from other Australian states.

Fig 2. Results from the SCRUB survey.

Left: for Round vi respondents (20–26 July) in Victorian Phase iv brake areas, reported frequency of mask use in the past 7 days. Numbers on pinnacle of confined represent total respondents. Right: the percentage of respondents who reported ever or often wearing a mask over time, Victorian Phase 4 restriction areas and compared to major cities in other states. The number of respondents varied from survey to survey and land to state but averaged 38.6 for each country and time betoken and the details are listed in S1 File.

https://doi.org/10.1371/periodical.pone.0253510.g002

Investigation of confounding variables

The proportion of non-metropolitan daily cases remained low (approximately five%) across the period of interest (July 10 –August x, 2020) and paralleled total Victorian cases. The proportion of daily cases in health care workers classified as caused in health intendance and aged care facilities increased steadily, only in that location was no significant change in the case growth rates across the pre- and mail-mask periods. Subtracting rural cases or cases in healthcare workers from the total cases made no significant difference to the calculated growth constants (S1 File).

There was a significant subtract in the number of tests per day performed over the study menses, with approximately 33% fewer tests on the last solar day, 10^th August, than the offset, ten^th July (linear regression: intercept 29,496; slope -310; p = 0.026). In addition, the pre- and postal service-mask period growth constants for the exponential increase and decrease in positive test ratio were college than the respective growth rates for cases. However, the difference in the growth rates for pre- and post-mask were similar for cases and for the positive test ratio (S1 File). The proportion of cases that were assigned to known clusters within 24 hours was approximately constant throughout the analysis catamenia.

The mobility information showed no significant change: 14.0% mobility pre-mask and 13.i% mail-mask (p = 0.09) relative to mobility in Feb 2020, prior to the onset of the Australian COVID-19 epidemic (Fig 1).

Including daily tests and the mobility alphabetize as boosted covariates in the multivariate linear regression made niggling difference to the estimated growth rates and neither the coefficient for mobility (-0.034, p = 0.28) nor tests (6 x 10−^{half dozen}, p = 0.36) were significantly different to zero (S1 File).

Changes to daily temperature could affect on virus transmissibility. Nonetheless, data supplied by the Australian Bureau of Meteorology for Olympic Park in the centre of Melbourne over the observation period, failed to show any significant trend over time (S1 File).

Discussion

Nosotros assessed growth rates in daily COVID-19 diagnoses in Victoria, Australia earlier and after the introduction of a mandatory mask policy in Melbourne (where 95% of Victoria'southward diagnosed cases occurred), finding a highly significant decrease of 0.065 per twenty-four hour period in the growth rate, from 0.042 per solar day to -0.023 per twenty-four hours. This decrease in growth rate is consistent with a change from an epidemic that is exponentially increasing with a doubling time of 16.5 days, to an epidemic that is exponentially decreasing with a halving fourth dimension of 30 days. This also corresponds to with a change in the estimated pre-mask range of R _eff from about 1.16 to 1.28 (assuming a series interval of 4 to vi.5 days) [29, thirty] to a post-mask range of R _eff from about 0.91 to 0.88. The critical outcome is that introduction of masks was associated with a alter from an increasing to a decreasing number of daily cases of COVID-19. This observation is consistent with modelling of the bear on of mask usage. This model also predicts that usage of mask fabricated the critical difference between the epidemic growing and contracting [32]. This took place in the context of existing stringent command measures and it is unclear what issue the masks would take had in the absenteeism of these other measures. An important characteristic of this study is an observed, rapid and substantial increase in the employ of masks within the population after 22^nd July 2020, as demonstrated in both self-study data and photographic data, that preceded a significant change in the growth rate of the epidemic. The rapid transition in the adoption of mask usage, based on photographic evidence, was hitting: from 43% before the declaration of the upcoming mandatory mask policy, to an boilerplate of 74% in a three-24-hour interval transition flow between proclamation and policy introduction, to near-complete compliance (98%) subsequently policy implementation. The photographic data was complemented by self-report survey data that showed consequent rapid change, from simply over 40% of participants reporting always or often wearing a mask on July 20, to 100% reporting always wearing a mask on 26 July (Fig 2).

Methodological strengths and weaknesses

The introduction of masks on tiptop of existing control program in Victoria provides an important guide of import opportunity to inform public health policy [33]. The of import strengths of this Melbourne study are

• Stable exponential growth in the daily cases prior to introduction of masks and no other interventions during the report menstruum; and
• A major and measured change in mask usage to close to full coverage of 12 and older people in public areas; and
• High daily testing rates for SARS-CoV-2 infection (average of 370 per 100,000 in Victoria) and depression positive test rates (average of 1.6% during the study), suggesting that most cases (although not all) would be detected.

We measured the overall impact associated with the introduction of mandatory mask use on daily cases. Care should exist taken in ascribing causality. We cannot decide whether masks had a direct effect or whether near-universal adoption of masks reminded wearers to engage in other behaviours recommended to reduce SARS-CoV-2 transmission, including regular hand hygiene and physical distancing. Interestingly, introduction of masks in Melbourne coincided with a decrease in the growth charge per unit in rural areas, where masks were not introduced, which is consequent with an indirect event, a decrease in seeding of rural areas from the Melbourne, or both.

An assumption was needed regarding the time-lag betwixt the implementation of a policy and observing its impact on detected cases. Nosotros used a time-lag of eight days after introduction of the mandatory mask policy, which is biologically plausible given the mean generation intervals reported and consistent with previous analyses in Melbourne and resulted in a model with a high goodness of fit.

Participants in the SCRUB Report were meant to written report their mask apply in the previous 7 days, only the rapid alter in behaviour over 20–26 July suggests participants interpreted this question as referring to their mask use on the twenty-four hour period they responded to the survey. There were also only 18 responders in the last three days of the survey that again limits the precision of the postal service-mask estimate, but it does back up the modify in mask usage judged from the larger photographic survey. Subsequent surveys of 1313 Victorians (thirteen to 18 August) and 994 Victorians (31 Aug to three September) reported 93.six% and 93.4%, respectively, that they "always" used a mask.

Other factors may have contributed to the decline in SARS-CoV-2 transmission at the fourth dimension the mandatory mask policy was introduced. Melburnians may have reduced their overall move in response to other official messages or how they interacted with each other, such equally whether people physically distanced from each other. Examination of mobility information across the relevant time period suggests mobility was stable [34, 35], although unpublished behavioural data collected through July suggests people were less likely to interact with other people as the month wore on, and when they did, kept physically distanced (pers comm., J McCaw, Doherty Institute). It is hard to know whether this change in interaction and concrete distancing would increase or decrease the effect nosotros observed. The decrease in interactions/contact may likewise have afflicted case numbers merely the refuse in interactions was steady over fourth dimension, and then cannot account for the hinge consequence we observed. Moreover, reduced interaction could hateful that the mask policy was more than constructive than measured, because masks were still effective despite potentially fewer episodes of risky contact. Changes to the fourth dimension needed for contact tracing may accept influenced the event; if the time taken from identifying a case to contacting and isolating all their contacts barbarous essentially during the flow in question, it may have created its own reduction in transmission. However, the proportion of cases assigned to a known cluster inside 24 hours, and unpublished data from the Victorian DHHS (pers comm., B Sutton, DHHS Victoria), suggests the time taken to perform contact tracing was stable across the menstruation of involvement.

To make the analysis every bit robust as possible, we did not exclude health care workers classified as having acquired COVID-19 in health care and aged care facilities from the assay, despite increasing numbers of cases in this group over the study menstruum, reflecting the loftier occupational risk of infection in these settings. However, subtracting cases of health care workers from the Melbourne totals made little difference to the overall growth constants. Health care workers were wearing extensive personal protective equipment (PPE), including masks, in their work environment before mandatory mask policy implementation. When the health intendance worker data were analysed separately, there was no significant alter in growth rates earlier and after the hinge mean solar day (S1 File) and there is no reason to believe that mask usage changed in the health care setting during the study period. These lack of changes in the health care worker cases supports the case that the changes observed in the general population are due to increased mask wearing in that population and not some general environmental factor.

We did not accept admission to daily case numbers for residents of aged intendance facilities over the whole of the assay period, but the bachelor data suggests that the cases in this group were growing rapidly during this time. Again, staff in aged care facilities were using PPE, including masks, extensively prior to masks condign mandatory in the full general community. This would suggest that the introduction of the mandatory mask policy would have less of an bear upon in this setting, given masks were already in apply, hence leading to an underestimate of the overall impact of masks. Availability of more than detailed example numbers over the whole Phase iii and Stage 3+ masks period would let this possibility to be tested rigorously. In addition, our assay did not take business relationship of transmission of COVID-19 in settings where mask use was not required, such equally households or some workplaces. Given very high transmission within households [xxx], again, this would lead to an underestimation of the impact of the widespread apply of masks.

Conclusions

Our results provide strong evidence to support that the policy of mandatory face masks was effective in reducing COVID-19 cases in a Melbourne with restrictions already in identify, accelerating the reduction of SARS-CoV-2 transmission. For the reasons outlined in a higher place, we have probably underestimated the bear on of masks in the community more broadly. Our work likewise indicates a loftier level of compliance with the Victorian Regime's policy, and the rapid increase in mask use following a mandate. Confront masks, whilst somewhat inconvenient for the private user, are less likely to take unintended negative impacts on the broader community than policies restricting movement, social engagement and the operations of business organization, schools and childcare. While the find change in growth rates were specific for the situation in Melbourne in mid 2020, never-the -less, our work strongly supports consideration of the use of face masks in other settings in Australia and globally to reduce community transmission of SARS-CoV-2.

Supporting information

Acknowledgments

Burnet Institute: Suman Majumdar and Ben Coughlin and Scott Umali–full general guidance and overview of literature; Caitlin Symon–review of photographs; Amanda Rotberg–development of figures; Bridget Draper–formatting of the manuscript; Campbell Aitken–manuscript editing.

Age paper: Michelle Stillman for review and provision of photographs.

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