Universal mask-wearing can reduce SARS-CoV-2 transmissibility by almost two-thirds, study finds

Universal mask-wearing can reduce SARS-CoV-2 transmissibility by almost two-thirds, study finds

By Dr. Liji Thomas, MDNov 17 2020

The coronavirus disease 2019 (COVID-19) pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) pathogen, continues to exert a devastating effect on global health and economic wellbeing late into 2020.


This scanning electron microscope image shows SARS-CoV-2 (yellow)—also known as 2019-nCoV, the virus that causes COVID-19—isolated from a patient in the U.S., emerging from the surface of cells (blue/pink) cultured in the lab. Image captured and colorized at NIAID's Rocky Mountain Laboratories (RML) in Hamilton, Montana. Image Credit: NIAID / Flickr

Without any approved vaccines or effective antivirals in sight, non-pharmaceutical interventions (NPI) aimed at curbing SARS-CoV-2's spread, such as social distancing and mask-wearing, remain the dominant approach across the globe.

A new study presents more empirical evidence that universal masking can reduce transmissibility by almost two-thirds, even with inadequate social distancing. The study was published on the preprint server medRxiv* in November 2020.

Though a couple of promising vaccines are entering their final stages of approval, it is unlikely that more than a small proportion of people will actually receive the vaccine in the near future. Thus, the need to contain the epidemic by other means remains vital.

However, the earliest measures used to prevent viral spread – such as national or regional lockdowns and/or school and workplace closures – are not easy to sustain for more than a few months at a time. Bans on non-essential businesses, large gatherings and social distancing in public, are also not enforceable or economically feasible for long periods.

Controversy over face masks

Face masks appear to be an easy but poorly accepted solution. In many places across the United States, there has been considerable controversy as to whether masking should be mandatory, or even advised, for any but healthcare workers or those who deal with the public. This disagreement has been fueled by contradictory messages from public health authorities such as the Centers for Disease Control and Prevention (CDC) and the governments at state and federal level.


Study: Association of social distancing and masking with risk of COVID-19. Image Credit: PERO studio / Shutterstock

However, evidence is pouring in that mask-wearing does effectively block transmission of the virus – much of it being derived from models constructed using data from community testing, hospitalization and mortality statistics. However, these do not give consideration to personal risk factors or the lag between the adoption of NPIs (mask-wearing being one) and rates of infection, as expected from the delay between symptom onset and testing or hospitalization.

Testing COVID-19 risk against social distancing

The researchers in the current study used individually collected data via COVID Symptom Study, a free smartphone app, where people report their symptoms, use of masks and other personal risk factors. This app was developed by the company Zoe Global Ltd. In collaboration with Massachusetts General Hospital, USA, and King's College London, UK.

The researchers combined this data with the frequency of social distancing observed in the community. This set of data was obtained by Unacast, where GPS was used to estimate social distancing compliance levels for each county.

This was utilized by the researchers to provide three mobility comparisons; namely, the average distance traveled per device, the number of non-essential visitations per device, and the number of close human-human encounters, as gauged by two devices within 50 m or less of each other, for 1 hour or less.

Between March 29 and July 16, 2020, the researchers used data from around 198,000 people. There were around 4,500 cases of COVID-19 predicted to occur in over 11 million person-days of follow-up.

They used predicted COVID-19 risk rather than positive COVID-19 test results because the number of app users who tested positive was small, perhaps due to poor testing availability. In fact, less than a fifth of infections in the US in March 2020 were detected, shows a recent study. Another multi-center study shows that the actual number of infections exceeded the reported number by 6-24 times, depending on the site, between March 23 and May 12.  

Future studies should explore the link between social distancing in a community with a higher testing prevalence and the risk of a positive COVID-19 test.

Communities without significant social distancing differed from those with higher social distancing, mainly in age, increased odds of current smoking, less lung disease, and increased interactions with confirmed or suspected COVID-19 positives, in the former.

Reduced predicted risk with increased social distancing

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On any given day, after a day on which social distancing was known to have been practiced, the risk of predicted COVID-19 was lower than otherwise. The maximal benefit was observed at 14 or more days afterwards.

Compared to communities with poor social distancing, the risk for predicted COVID-19 cases was reduced by 15%, 20% and 31% with fair, good and excellent social distancing, respectively, at 14 days. Increasing social distancing from poor to excellent was most beneficial among middle-aged people (36-55 years), reducing the predicted risk by more than 50%. The risk reduction was independent of the phase of the epidemic.

However, there was no observed reduction in risk among individuals staying at home or who regularly used mobility aids, probably because they were already less likely to go out in public.

The risk was also lower by 22% when the average distance traveled was reduced to less than 25% compared to more than 55% of the original. With less than 55% non-essential visitations compared to over 65% of the original, there was, again, a reduction in predicted COVID-19 risk by 21%.

Close human encounters failed to show any association with lowered risk, which may indicate that this method is not a good measure of social distancing, or indeed of SARS-CoV-2 transmission.

Reduced risk with masking

With mask use, the predicted risk among those who sometimes, mostly or always used a mask was 65% less compared to those who never used a mask. When categorized by the type of social distancing in the community one lived among, the risk for constant wearers fell by around 70% and 63% with excellent, good or fair social distancing, and poor social distancing, respectively.

Remarkably, these results were robust to adjustment for COVID-19 incidence, and for earlier relaxation of social distancing, as well as for the reproduction number Rt.

The researchers say, "These findings imply that baseline risk did not impact the relative benefits of social distancing policies and/or face mask use, although it remains possible that the absolute reduction in risk is greater in areas with higher burden of COVID-19."

Earlier research

The authors comment, "Our findings are consistent with previous ecological studies investigating the effect of social distancing on risk of COVID-19." For instance, each unit of increase in social distancing reduces COVID-19 incidence and mortality by a third. Again, the rates of the epidemic's growth fell by about 1% from four days after the beginning of state-wide social distancing practices.

Several other studies show higher case burdens in Iowa, USA, one of the few American states which never issued a shelter-in-place order, compared to Illinois, USA, which did, and saw a 13% fall in incidence without any physical distancing in place.

Mask-wearing among all healthcare workers in hospitals has reduced COVID-19 rates, says another study. Also, universal masking is believed to reduce the risk of other betacoronavirus infections among both healthcare workers and others –  particularly severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS).

The current study also details how mask use can protect both the wearer and others. Of course, this effect could also be because participants who typically wear masks may have healthier behaviors, overall.

What are the implications?

The study concludes, "We demonstrated a significantly reduced risk of predicted COVID-19 infection among individuals living in communities with a greater social distancing grade at 14 days. Among participants who lived in a community with poor social distancing, wearing a face mask was associated with reduced risk."

The study thus demonstrates the ability of NPIs to reduce the number of infections – most markedly, as the data suggests, at 14 days from intervention. The authors recommend, "Encouraging universal masking may be particularly important to limit the continued spread of COVID-19 as social distancing mandates continue to be relaxed."

These findings corroborate the importance of practical NPI compliance, like mask-wearing, in saving many vulnerable individuals' lives. As the world awaits an effective and widely vaccination that immunizes against SARS-CoV-2 and improved therapies to help treat COVID-19, these measures can make a big difference in ebbing the tide of the pandemic.

*Important Notice

medRxiv publishes preliminary scientific reports that are not peer-reviewed and, therefore, should not be regarded as conclusive, guide clinical practice/health-related behavior, or treated as established information.

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