1. Goodwin, Peter M.

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Powerful new evidence that face coverings could prevent the risk of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) transmission has emerged from an online survey among a third of a million Americans published in The Lancet Digital Health (2021; doi:

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"Our findings, based on observational data, suggest a community benefit for wearing face masks for slowing the transmission of COVID-19," said senior author of the study, John S. Brownstein, PhD, of the Computational Epidemiology Laboratory and Division of Endocrinology at Boston Children's Hospital.


Study co-author Ben Rader, MPH, also from Boston Children's Hospital, said another important finding of this research had been that mask wearing should not be viewed as a replacement for physical distancing. "U.S. states that practiced both at high levels had the best probability of controlling disease spread," he said.


Brownstein said that, although governments around the world had mandated mask-wearing in public spaces to try and stem the transmission of COVID-19 (by providing a physical barrier that prevented the spread of virus-laden droplets), a strong evidence-base for using face coverings had been lacking.


"Past evidence on the effectiveness of mask use against COVID-19 transmission is mixed. And setting up randomized controlled trials to investigate this is challenging," he told a press briefing.


Opinion (and some evidence) had suggested surgical masks and cloth masks were likely to help prevent wearers from passing on infection to others. But most publicly available face masks were not necessarily designed to protect wearers themselves from infection by meeting the standards required by medical units at high risk of infectious transmission (which are met by the N95 specification masks). The study offered a way of assessing the effectiveness of any face covering-without specifying which type-by comparing self-reported mask use with infection rates in different American states.


"We directly examined mask-wearing instead of using mask mandates as a proxy," wrote the investigators. "We supplemented case studies by using a large sample representative of the population across the USA, combined with community estimates of SARS-CoV-2 transmission."


Researchers conducting an online survey of 374,021 Americans (who had to be at least 13 years old) between June 3 and July 27, 2020, to ask them about their use of face masks in a variety of situations. Data from the responses were then plotted against estimates (from two publicly available sources) of the reproductive number, R (defined as the average number of people infected by each case). The researchers also noted potentially confounding variables-including physical distancing and community demographics. These data were used in a mathematical model to estimate the association between mask-wearing and community transmission control (defined as keeping R below 1.0). The researchers also noted mask-wearing data from 12 states 2 weeks before, and then again 2 weeks after, statewide mandates requiring face coverings in public places.


The survey gathered data from across the 50 states and the District of Columbia. Physical distancing within each state was estimated from the length of time people spent at home (using anonymized data from Google users who had opted to allow sharing of their location history on their mobile devices).


Among survey responses the investigators found "an increasing trend in reported mask usage across the U.S., "although uptake varied by geography. After controlling for variables such as physical distancing and population demographics, the analysis found that a 10 percent increase in self-reported mask-wearing was associated with a more than three-fold increase in the median odds of achieving transmission control (R below 1.0).


Although the data were heterogeneous, the study finding was clear, wrote the authors. "We showed that states with higher rates of mask-wearing had estimated reproductive numbers that were lower than the crucial threshold needed to maintain disease transmission."


But the study found no gain from mandating mask-wearing. Data analysis before and after the introduction of mandates in specific states indicated that "mask-wearing showed no statistically significant change in the slope." But the authors noted that neither did the mandates hold back the upward trend in reported mask-wearing (that had been observed during the entire 2-month study period).


"Mandates alone may not be enough to increase mask use, said Brownstein. "We recommend that policy makers consider additional strategies to evaluate and increase mask usage in order to disrupt the ongoing COVID-19 epidemic."


The researchers concluded that the widespread use of face masks they had observed (as reported by survey responses), combined with physical distancing, had together increased the odds of achieving SARS-CoV-2 transmission control.


The fact that self-reported mask-wearing increased separately from government mask mandates suggested that supplemental public health interventions were needed to maximize face-covering use to curb the ongoing epidemic.


"In combination with previous research, this study shows that the community adoption of face masks might be an important non-pharmaceutical intervention for the reduction of SARS-CoV-2," they wrote.


Co-author Christina M. Astley, MD, ScD, Attending Physician, and epidemiologist with Boston Children's Hospital and Harvard Medical School, said the findings suggested widespread use of face masks could help to control SARS-CoV-2 transmission.


"The world is facing a more transmissible coronavirus strain, hospitals are struggling with new cases, and vaccination programs are still being rolled out," Astley said. "Interventions are needed now to lower the burden on our health care systems. This research provides additional evidence that those interventions should include wearing face masks to protect ourselves as well as physical distancing."


Commenting in The Lancet, Hannah Clapham, PhD, of the National University of Singapore (who was not involved in the study), wrote that the evidence was clear: "Masks work!" However, their use was non-targeted. So it needed to be accepted by society as a whole if it were to be successful. She commented on the absence of any improvement in transmission control following the introduction of mandates.


"This suggests that in the U.S. there is a disconnect between rules, messaging and actions, and that further socio-behavioral research is needed on what motivates people who choose not to wear a mask to protect themselves and those around them," Clapham concluded.


Peter M. Goodwin is a contributing writer.