10% rise in mask-wearing triples chance of keeping coronavirus' R number below one, study suggests

Alexandra Thompson
·6-min read
Young woman wearing surgical mask in front of home
Face coverings play a role in stemming the coronavirus outbreak, research suggests. (Posed by a model, Getty Images)

A 10% rise in mask-wearing could triple the chance of keeping the coronavirus’ reproduction (R) number below one, research suggests.

Although recommended by the World Health Organization (WHO), the benefits of face masks or coverings in stemming the infectious outbreak have been hotly debated.

To learn more, scientists from Boston Children’s Hospital linked the results of a survey completed by more than 300,000 people to modelled estimates of viral transmission.

Results – published in The Lancet Digital Health – suggest combining social distancing with a 10% increase in self-reported mask-wearing more than triples a community’s odds of keeping R below one.

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R describes the number of people an infectious person is expected to pass the virus on to. For example, if R is two, every coronavirus patient statistically infects two others.

When R is below one, an outbreak declines.

Data linking dose to symptoms have been gathered for other microbes that attack the human airway, including influenza viruses and the bacteria that cause tuberculosis. (Getty Images)
Face coverings are most effective when combined with social distancing. (Posed by models, Getty Images)

“Wearing face masks or face coverings in public spaces has been mandated by governments around the world to try and stem transmission of COVID-19,” said study author Professor John Brownstein.

“The aim is to provide a physical barrier that prevents the spread of virus-laden droplets.

“However, 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.

“Our findings, based on observational data, suggest a community benefit for wearing face masks for slowing the transmission of COVID-19, however, mandates alone may not be enough to increase mask use.

“We recommend policy-makers consider additional strategies to evaluate and increase mask usage in order to disrupt the ongoing COVID-19 epidemic.”

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In the UK, face coverings are mandatory for most in public enclosed spaces, like supermarkets or on public transport.

Face coverings are intended to be more comfortable than medical-grade masks, aiding adherence, with the government even recommending people fashion their own out of squares of cotton.

The WHO has changed its stance on public face coverings during the pandemic.

The organisation previously said wearing a mask may prevent an infected individual from passing the virus on, but there was no evidence a covering would protect an uninfected person from catching the coronavirus.

With up to one in three coronavirus cases being asymptomatic, however, mandatory masks could help prevent these individuals unwittingly infecting others.

Laboratory studies have also shown face masks reduce the amount of virus detectable in a wearer’s breath.

A later trial also suggested, however, wearing a mask did not affect a user’s risk of infection.

Watch: How to wear a face covering comfortably

To learn more, the Boston scientists sent a survey to hundreds of thousands of people across the US between 3 June and 27 July 2020, during which time mask-wearing became mandatory in 12 states.

The participants were asked to rate on a four-point scale how likely they were to wear a mask while supermarket shopping or visiting family and friends.

Social distancing rates within each US state were estimated according to the amount of time people spent at home. This was based on anonymised data from Google users who allowed sharing of their location history on their mobile.

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The Google data was compared against that from 3 January to 6 February, before social distancing came into play.

The scientists collaborated this social distancing information against that of Carnegie Mellon University’s Delphi Research Groups COVID-19 symptom survey.

Virus transmission in each state was based on the daily estimated R number from the databases Rt.live, The COVID Tracking Project and the COVID-19 data working group.

‘The evidence is clear: masks work’

Results reveal more than four in five (84%) of the participants were “very likely” to wear a face mask to the supermarket, while just two in five (40%) did so when visiting family or friends.

Less than one in 20 (4%) said they were “not likely to wear a mask at all”.

The scientists found the communities with high self-reported mask-wearing and social distancing had the highest predicted probability of transmission control.

“Our findings suggest widespread use of face masks may help to control [coronavirus] transmission,” said co-author Dr Christina Astley.

“The world is facing a more transmissible coronavirus strain, hospitals are struggling with new cases and vaccination programmes are still being rolled out.

“Interventions are needed now to lower the burden on our healthcare systems.

“This research provides additional evidence those interventions should include wearing face masks to protect ourselves and as well as physical distancing.”

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Co-author Ben Rader added: “An important finding of this research is mask-wearing is not a replacement for physical distancing and [the] US states [that] practiced both at high levels had the best probability of controlling disease spread.”

The scientists stressed people who adhere to mask-wearing may also be more compliant with other coronavirus-related advice, like regular hand washing, which was not addressed in the study.

They also did not account for contact tracing or coronavirus testing, which may both play a key role in transmission control.

The type of face masks the participants wore was also not recorded. Ideally, a covering should consist of two layers, while the WHO recommends three layers for certain fabrics.

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Writing in The Lancet, Dr Hannah Clapham from the National University of Singapore – who was not involved in the study – said: “The evidence is clear: masks work.

“However, their use is a non-targeted control measure, wherein the whole population is involved, rather than just known or suspected cases.

“Buy-in from society as a whole is therefore necessary for the success of the intervention.”

The Boston scientists found mask-wearing increased steadily throughout the survey period, however, there was no notable rise when the coverings were made mandatory in the 12 states.

“This suggests in the US there is a disconnect between rules, messaging, and actions; and further socio-behavioural research is needed on what motivates people who choose not to wear a mask to protect themselves and those around them,” said Dr Clapham.

She stressed masks must not encourage complacency, for example thinking wearing a covering enables in-person socialising.

Nevertheless, masks offer a “cheap” and “low-risk” form of protection.

“On a policy level, the possibility of risk compensation, uncertainty about the magnitude of the reduction in transmission due to masks, and the result the effect of face masks can be even greater when combined with other control measures, [which] means maintaining other measures, such as physical distancing, contact tracing, testing, isolation, and quarantine, is still necessary,” added Dr Clapham.

Watch: How to remove a face covering correctly