Policies that increase prevalence of mask-wearing by 10% in a community would more than triple its likelihood of hitting a key COVID-19 control target, a modelling study suggested. After controlling for physical distancing, population demographics, and other confounders, the odds ratio for achieving an instantaneous reproductive number (R1) less than 1 for SARS-CoV-2 virus transmission was 3.53 (95% CI 2.03-6.43) for each 10% increment in mask-wearing, reported Dr John Brownstein, of Boston Children's Hospital, and colleagues.
"Past evidence on the effectiveness of mask use against COVID-19 transmission is mixed and setting up (randomised) controlled trials to investigate this is challenging," Brownstein said. "Our findings, based on observational data, suggest a community benefit for wearing face masks for slowing the transmission of COVID-19."
An accompanying editorial by Dr Hannah Clapham and Dr Alex Cook both of National University of Singapore, characterised the research as "an elegant ecological analysis." They acknowledged mask wearing has been divisive, but said this study helps to bring some clarity to the debate about whether masks protect the wearer from infection or prevent the wearer from transmitting the virus themselves.
"An ecological analysis … measures the overall effect of face mask wearing on transmission, and thus obviates the need to disentangle the two modes of effect," the editorialists wrote.
And while observational and laboratory studies are generally considered "weak evidence" for an intervention versus the "gold standard" randomised trial, Clapham and Cook noted any trial trying to prove mask wearing protects others from SARS-CoV-2 would be "logistically challenging, bordering on infeasible and potentially unethical."
"Well conducted, real-world, observational studies … probably provide the strongest evidence to inform policy," they wrote.
Brownstein and colleagues examined data from serial cross-sectional surveys administered via SurveyMonkey to randomly selected individuals ages 13 or older in the US from 3 June to 27 July. They were asked about mask usage on a four-point scale ranging from "very likely" to "not likely at all" on how likely they were to wear masks "while grocery shopping" or "while visiting with family and friends in their homes," as well as a range of questions about mask usage.
They analysed self-reported data with R1 estimates, or the number of secondary cases arising from a single case for a given day, from both the COVID Tracking Project and the open COVID-19 data working group, to estimate transmission control (defined as R1<1). They then estimated the association between mask wearing from the survey data and community transmission control.
Data from 378,207 survey responses were included. About 85% of individuals said they were very likely to wear a mask to the grocery store, while 40% said they would wear one with friends and family. Self-reported mask wearing was higher among women, respondents with a lower income, and Black, Hispanic, and other racial/ethnic groups versus white individuals. The highest percentage of mask wearing occurred along the US coasts and southern border and in large urban areas.
Distancing played an important role in the modelling results as well, showing that states with high mask wearing – but no change from baseline in physical distancing – had a 22% predicted probability of community transmission control, albeit with a wide confidence interval (3%-76%).
"Self-reported mask-wearing is shown to increase the odds of transmission control across all levels of physical distancing, suggesting that any intervention to improve this community-based behaviour might be worthwhile," Brownstein and colleagues wrote.
Study details
Mask-wearing and control of SARS-CoV-2 transmission in the USA: a cross-sectional study
Benjamin Rader, Laura F White, Michael R Burns, Jack Chen, Joseph Brilliant, Jon Cohen, Jeffrey Shaman, Larry Brilliant, Moritz U G Kraemer, Jared B Hawkins, Samuel V Scarpino, Christina M Astley, John S Brownstein,
Published in The Lancet Digital Health on 19 January 2021
Abstract
Background
Face masks have become commonplace across the USA because of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) epidemic. Although evidence suggests that masks help to curb the spread of the disease, there is little empirical research at the population level. We investigate the association between self-reported mask-wearing, physical distancing, and SARS-CoV-2 transmission in the USA, along with the effect of statewide mandates on mask uptake.
Methods
Serial cross-sectional surveys were administered via a web platform to randomly surveyed US individuals aged 13 years and older, to query self-reports of face mask-wearing. Survey responses were combined with instantaneous reproductive number (Rt) estimates from two publicly available sources, the outcome of interest. Measures of physical distancing, community demographics, and other potential sources of confounding (from publicly available sources) were also assessed. We fitted multivariate logistic regression models to estimate the association between mask-wearing and community transmission control (Rt<1). Additionally, mask-wearing in 12 states was evaluated 2 weeks before and after statewide mandates.
Findings
378 207 individuals responded to the survey between June 3 and July 27, 2020, of which 4186 were excluded for missing data. We observed an increasing trend in reported mask usage across the USA, although uptake varied by geography. A logistic model controlling for physical distancing, population demographics, and other variables found that a 10% increase in self-reported mask-wearing was associated with an increased odds of transmission control (odds ratio 3·53, 95% CI 2·03–6·43). We found that communities with high reported mask-wearing and physical distancing had the highest predicted probability of transmission control. Segmented regression analysis of reported mask-wearing showed no statistically significant change in the slope after mandates were introduced; however, the upward trend in reported mask-wearing was preserved.
Interpretation
The widespread reported use of face masks combined with physical distancing increases the odds of SARS-CoV-2 transmission control. Self-reported mask-wearing increased separately from government mask mandates, suggesting that supplemental public health interventions are needed to maximise adoption and help to curb the ongoing epidemic.
[link url="https://www.medpagetoday.com/infectiousdisease/covid19/90765"]MedPageToday report (registration needed)[/link]
[link url="https://www.thelancet.com/journals/landig/article/PIIS2589-7500(20)30293-4/fulltext"]The Lancet study (Open Access[/link]
[link url="https://www.thelancet.com/journals/landig/article/PIIS2589-7500(21)00003-0/fulltext"]The Lancet comment (Open Access)[/link]