coronavirus

More Evidence for Masks—and Why They’re Not Enough on Their Own

A 10-percent increase in self-reported mask wearing is associated with a more than three-fold increase in chances of a community having coronavirus transmission under control. But communities were best off with both more mask wearing and more physical distancing.

Nearly a year into the US COVID-19 epidemic, mask wearing is perhaps the most visible and increasingly politicized symbol of the coronavirus era.

Now, a new study co-authored by School of Public Health and Boston Children’s Hospital researchers adds to the evidence supporting mark wearing.

Published in The Lancet Digital Health journal, the modeling study finds that a 10-percent increase in self-reported mask wearing is associated with a more than three-fold increase in the chances of a community having coronavirus transmission under control.

However, the study also found that mask mandates did not appear to be enough to markedly increase mask wearing, and that communities with the best handle on COVID were the ones with both high levels of mask wearing and more physical distancing.

“The results of our manuscript suggest that face masks do work and that we need innovative solutions to increase their usage around the US,” says study first author Benjamin Rader, a doctoral student in epidemiology at SPH and a graduate research fellow in the Computational Epidemiology Lab at Boston Children’s Hospital.

“I am hopeful that novel initiatives, like President-elect Biden’s 100-day mask plan, can increase facemask wearing and curb the spread of SARS-CoV-2 around the country,” he says.

At the same time, Rader says, “an important finding of this research is that mask wearing is not a replacement for social distancing, and US states that practiced both at high levels had the best probability of controlling disease spread.”

Study co-author Laura White, associate professor of biostatistics, says the study adds to an ever-growing body of research showing that masks help prevent the spread of COVID. “My hope is that this serves as a reminder and reinforcement of the need to be vigilant in wearing masks,” she says. “I think that we often need reminders and continued evidence that supports behaviors that we can implement now to stay safe and protect others. Given the large scope of this study and extensive data we had to work with, this is a very strong demonstration that not only do masks work well in controlled research settings, but that we have strong evidence that they are working in non-research settings and can substantially limit transmission.”

The new study used mathematical modelling to investigate the association between self-reported mask-wearing, physical distancing, and SARS-CoV-2 transmission in the US through June and July of 2020.

The researchers used a web-based survey to gather self-reported data on mask-wearing habits from 378,207 people in all 50 states and the District of Columbia. Participants rated on a four-point scale how likely they were to wear a mask while grocery shopping or visiting family and friends.

The team estimated physical distancing within each state based on the amount of time people spent at home according to anonymized data from Google users who had opted to allow sharing of their location history on their mobile devices. The researchers compared data from June and July and a baseline period of January 3 through February 6, 2020—back before people changed their behaviors because of the pandemic.

They estimated levels of virus transmission in each state over time based on the daily reproductive (R) number from two publicly-available databases. The R number represents the estimated number of people one infected person will go on to infect, so an R number below 1 means that infections are in decline.

Controlling for physical distancing rates, and other variables including population density and demographics, the researchers found that a 10 percent increase in self-reported mask-wearing was associated with a 3.53-fold increase in the likelihood that a community’s R number stayed below 1.

Mask-wearing rates increased steadily throughout the survey period, but the authors did not find an additional increase following the introduction of government mandates. The researchers write that this may explain why studies analyzing mask mandate interventions may find mixed results despite the potential benefits of mask wearing.

“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,” says study senior author John Brownstein, director of the Computational Epidemiology Group at Boston Children’s Hospital and professor of pediatrics at Harvard Medical School.

“Our findings, based on observational data, suggest a community benefit for wearing face masks for slowing the transmission of COVID-19,” he says. “However, mandates alone may not be enough to increase mask use. We recommend that policymakers consider additional strategies to evaluate and increase mask usage in order to disrupt the ongoing COVID-19 epidemic.”  

“The world is facing a more transmissible coronavirus strain, hospitals are struggling with new cases, and vaccination programs are still being rolled out,” says study co-author Christina M. Astley, a clinician and epidemiologist with Boston Children’s Hospital and Harvard Medical School. “Interventions are needed now to lower the burden on our healthcare systems. This research provides additional evidence that those interventions should include wearing face masks to protect ourselves and as well as social distancing.”