COVID-19 Resource Center
COVID-19 Resource Center
Perspective from Monica Gandhi, MD, MPH
Source/Disclosures
Disclosures: Bundgaard, Cash-Goldwasser and Frieden report no relevant financial disclosures. Laine reports being editor in chief of Annals of Internal Medicine and senior vice president at the American College of Physicians. Please see the study for all other authors’ relevant financial disclosures.
November 18, 2020
3 min read
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Study: COVID-19 risk slightly lower for mask wearers

Perspective from Monica Gandhi, MD, MPH
Source/Disclosures
Disclosures: Bundgaard, Cash-Goldwasser and Frieden report no relevant financial disclosures. Laine reports being editor in chief of Annals of Internal Medicine and senior vice president at the American College of Physicians. Please see the study for all other authors’ relevant financial disclosures.
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The CDC recently updated its guidance to state that face masks provide some personal protection against COVID-19 for those who wear them, not just for those around them.

New data from a randomized controlled study published in Annals of Internal Medicine showed that wearing a mask was slightly more effective in preventing COVID-19 than not wearing a mask in situations where other preventive measures like physical distancing are recommended.

Respirators
The CDC recently updated its guidance to state that face masks provide some personal protection against COVID-19 for those who wear them, not just for those around them. Source: Adobe Stock.

Henning Bundgaard, MD, DMSc, a professor and consultant in cardiology at the University of Copenhagen and coauthor of the study, told Healio Primary Care that it is important to note that the researchers only assessed the effect of masks as personal protection — not “source control.”

“The identified potential positive effect of the mask for prevention — although small — might still be of interest considering COVID-19 is a very serious disease,” he added.

Photo of Henning Bundgaard
Henning Bundgaard

Bundgaard and colleagues conducted a randomized controlled study in Denmark from April 3 to June 2, where participants who spent at least 3 hours per day outside of their home were randomly assigned to wear a mask or to not wear a mask when away from home.

Those asked to wear a mask were given 50 high-quality surgical face masks with a 98% filtration rate.

At the time of the study, Danish authorities did not recommend the use of masks, and less than 5% of people outside of hospitals used masks, according to the researchers. However, authorities recommended other preventive measure during the study period such as quarantining people with COVID-19, physical distancing measures, limiting contact with people, frequent handwashing and cleaning and limiting visitors in hospitals and nursing homes. Denmark’s cafes and restaurants were closed from the start of the study period until May 18.

Among participants, the researchers evaluated the prevalence of SARS-CoV-2 infection at 1 month based on positive antibody results, polymerase chain reaction or diagnosis at a hospital.

A total of 3,030 participants were assigned to wear masks and 2,994 were assigned to not wear masks.

Bundgaard and colleagues found that 1.8% of those who wore masks and 2.1% of those who did not wear masks tested positive for SARS-CoV-2 infection. They determined that the difference between the groups was -0.03 percentage points (OR = 0.82; 95% CI, 0.54-1.23), with the findings in favor of those who wore masks.

The researchers noted that additional analyses accounting for participants with missing follow-up data yielded similar results.

They said that while the difference between the two groups was not statistically significant, the data were “inconclusive,” with mask wearing demonstrating a potential 46% decrease to a 23% increase in COVID-19.

Bundgaard said that previous observational studies have shown that masking is effective, but the studies are challenging to interpret due to the limitations of observational data. However, he added that laboratory research has shown that wearing masks appears to be effective.

He noted that additional randomized studies on mask wearing will be difficult to perform because face masks are mandatory in most places.

In an editorial accompanying the study, Christine Laine, MD, MPH, editor in chief of Annals of Internal Medicine, and colleagues stressed that the findings do not address the effectiveness of masks in preventing transmission in areas were most people mask, and that the findings do not disprove the effectiveness of masking.

Laine and colleagues said CDC guidance recognizes that mask wearing can reduce transmission when worn by everyone. They added that the study by Bundgaard and colleagues suggests that when masks are worn by only a minority of people, those who wear them are not “invulnerable” to COVID-19, and that the impact of mask wearing on personal protection is probably small.

“All who worry about the COVID-19 pandemic should carefully consider these findings for what they show and refrain from viewing them as evidence that widespread mask-wearing is ineffective,” they wrote. “While we await additional evidence about the effectiveness of masks as source control of SARS-CoV-2 transmission, we are going to do our part to protect everyone by masking up and hope that those around us do the same.”

In another editorial published alongside the study, Former CDC Director Thomas R. Frieden, MD, MPH, and Shama Cash-Goldwasser, MD, MPH, of Resolve to Save Lives, an initiative of Vital Strategies in New York, highlighted several limitations of the study, including that it was conducted in an area with relatively low transmission and that antibody tests were used to diagnose COVID-19.

“Although no single strategy can control the pandemic, widespread masking in the community can mitigate spread as part of a comprehensive approach,” they wrote.

Frieden and Cash-Goldwasser added that “if everyone wears a mask when near others, everyone is safer.”

References:

Bundgaard H, et al. Ann Intern Med. 2020;doi:10.7326/M20-6817.

CDC. Scientific brief: Community use of cloth masks to control the spread of SARS-CoV-2. https://www.cdc.gov/coronavirus/2019-ncov/more/masking-science-sars-cov2.html. Accessed Nov. 18, 2020.

Laine C, et al. Ann Intern Med. 2020;doi:10.7326/M20-7448.

Frieden TR, et al. Ann Intern Med. 2020;doi:10.7326/M20-7499.