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A sign asking customers to wear a face covering is displayed at the entrance to Picos restaurant Wednesday, March 10, 2021, in Houston. (AP Images) A sign asking customers to wear a face covering is displayed at the entrance to Picos restaurant Wednesday, March 10, 2021, in Houston. (AP Images)

A sign asking customers to wear a face covering is displayed at the entrance to Picos restaurant Wednesday, March 10, 2021, in Houston. (AP Images)

Noah Y. Kim
By Noah Y. Kim March 12, 2021

If Your Time is short

  • Because of ethical considerations, scientists haven’t been able to conduct randomized controlled trials to conclusively prove the effectiveness of mask-wearing on interrupting  coronavirus transmission.

  • However, a wealth of other studies and the totality of scientific research shows that mask-wearing is effective at preventing the spread of the coronavirus. ​

Nearly a year since the CDC first started recommending wearing face coverings in public, social media users are still denying their efficacy. 

"Every real world, randomized controlled trial on mask effectiveness demonstrates that masks are not effective in reducing the spread of viral infections," says author Shawn Stevenson in an Instagram video

The post was flagged as part of Facebook’s efforts to combat false news and misinformation on its News Feed. (Read more about our partnership with Facebook.)

A randomized controlled trial is a scientific study that randomly sorts participants into an experimental group and a control group to study the effect of a particular variable on the outcome. Randomized controlled trials are time-consuming and expensive, but well-designed ones are more effective at minimizing bias than most other types of studies. 

So is Stevenson correct in saying that there are no randomized controlled trials demonstrating the effectiveness of mask use in blunting viral infections? No, he’s not. Multiple randomized controlled trials have shown that mask-wearing effectively blunts the spread of other coronaviruses and influenza-like diseases.

And Stevenson’s assertion that no randomized controlled trials have conclusively proven that masks prevent the spread specifically of Sars-CoV-2 ignores that there is no lack of scientific proof establishing the efficacy of mask wearing. It also very notably misleads by leaving out the reason why randomized controlled trials have not been used for this purpose: in the midst of a pandemic, employing these methods to study the question would widely be considered unethical.  

We reached out to four public health experts and asked them to sum up the scientific evidence on the effectiveness of mask use. They provided scores of systematic reviews, ecological studies and laboratory studies showing that masks play an important role in reducing the spread of the coronavirus and play a critical role in interrupting viral transmission. 

Stevenson did not return a request for comment. 

Why are there so few randomized controlled trials on mask use?

To conduct a randomized controlled trial on the efficacy of masks, researchers would have to randomly assign some members of a community not to wear a face mask for a long period of time to see whether they got sick at higher rates than a control group.

Practically there is no way that scientists could run a study like this during a global pandemic without endangering trial participants and other people they encountered out in the world.

"Randomized controlled trials are pretty much the gold standard, but they’re not always ethical," said Mary Kathryn Grabowski, an assistant professor in epidemiology at Johns Hopkins University. "We can’t just send people out without masks in the middle of a pandemic in the same way we can’t randomize people to not use a parachute when they jump out of a plane." 

According to Babak Javid, a professor of medicine at the University of California, San Francisco, only one randomized controlled trial of mask use has been conducted during the COVID-19 pandemic. This study attempted to show whether volunteers who were given and taught to wear surgical masks were more protected from the coronavirus than those who weren’t. However, the study didn’t analyze source control –  the ability of masks to prevent infected wearers from spreading the virus to other people – which is the primary reason why scientists believe masks are effective at interrupting transmission. 

This study, known as DANMASK-19, adhered to ethical guidelines because it was conducted in Denmark early in the pandemic while community spread of the coronavirus was at only 2%. However, that also limited its scope and precision, said Johns Hopkins epidemiologist Nikolas Wada. 

Beyond the fact that the study was underpowered, it had other limitations, said Wada. Huge numbers of trial participants did not comply with their directions, with less than half the people in the masked group reporting that they wore face coverings as instructed. In addition, the study was conducted at a time when other public health guidelines such as social distancing were being implemented, making it difficult to tell what protection came from the masks and what came from these other measures. 

Despite these imperfections, the study found that those who wore masks were 18% less likely to be infected than those who did not. However, since the study was only designed to detect a large effect greater than 50%, the 18% difference was statistically insignificant, meaning that it could have happened by chance.

Some social media users such as Stevenson cite the DANMASK-19 study as evidence that masks don’t work. However, Wada said, this is actually a common misinterpretation of the study. 

"Basically, you couldn't draw up a better design to show no mask benefit, but there appears to have been a benefit anyway. And that's just to protect the wearer, without any measurement of source control," said Wada.

Why do we know that masks are effective?

No scientific study is perfect. Instead of basing their opinions off individual trials, public health experts look beyond isolated articles to see whether the totality of research points in one direction or the other. 

"I think taking individual studies on masks and using them to make a point without considering the totality of the literature is a really bad idea," said Grabowski. "It’s important to consider all of the data and all of the research on this topic."

Building off that principle, Grabowski and her colleagues at Johns Hopkins have compiled a list of significant scientific studies on COVID-19 and analyze the findings and limitations of each. 

Due to the lack of randomized controlled trials on mask use’s effectiveness against COVID-19, public health experts have based their guidance on a variety of other scientific studies: systematic reviews, ecological studies and laboratory studies. 

Systematic reviews are papers that pool existing studies and try to answer a narrowly defined question using a larger data set than any of the studies had individually. According to scientific hierarchies of evidence, systematic reviews are generally thought to produce the most reliable evidence. 

A systematic review on mask-use published in the Lancet analyzed 172 observational studies across 16 countries and six continents and found that "Face mask use could result in a large reduction in risk of infection … with stronger associations with N95 or similar respirators compared with disposable surgical masks or similar." However, many of the studies included in the review took place in healthcare settings rather than in community settings. 

Another systematic review on the ability of mask-use to interrupt the spread of respiratory illnesses, which included 67 randomized controlled trials and observational studies, found that "overall masks were the best performing intervention across populations, settings and threats."

There have also been a wealth of ecological studies on masks and COVID-19. These studies analyze the effect that specific localized public health measures have on coronavirus case rates after they’re implemented. These papers have consistently found that mask mandates cause sharp declines in coronavirus case rates. 

One of these papers, which analyzed 15 U.S. states found that daily infection rates decreased significantly after mask mandates were introduced. In seven of the 15 states, researchers from UC San Diego and Texas A&M found that the number of new infections per day increased steadily only to fall after face-mask requirements were implemented. In the six states the researchers analyzed that never implemented mask mandates, infections continued to increase on the same upward line. They estimated that mask mandates had prevented a total of 252,000 infections on May 18, equivalent to nearly 17% of infections in the nation at that point in time. 

Finally, scientists have studied the spread of coronavirus particles in laboratory settings and found that masks are effective at stopping them from dispersing into the air, suggesting that they also function that way in real world settings. 

It’s important to note, said Javid, that mask-wearing shouldn’t be viewed as a panacea for COVID-19 control. Some people who wear masks will get sick or spread the virus to others. The best scientific evidence shows that masks help to prevent viral spread, but that their effectiveness varies based on the quality of the mask worn and the strictness with which people adhere to mask-wearing guidelines. 

"Unless one wears a properly fitted respirator mask for all interactions​ outside of the household, masks cannot, by themselves, completely interrupt transmission," he said. "Nonetheless, from what we know about the biology of the disease, and the biophysics of droplet and aerosol production, there is a strong rationale for why masks can have some effect in both protecting the wearer and as source control." ​

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Our Sources

Interview with Mary Kathryn Grabowski, an assistant professor in epidemiology at Johns Hopkins University, Mar. 10, 2021

Email interview with Nikolas Wada, an epidemiologist at Johns Hopkins University, Mar. 10, 2021

Email interview with Maureen Ferran, an associate professor of biology at the Rochester Institute for Technology, Mar. 11, 2021

Email interview with Babak Javid, a professor of medicine at the University of California, San Francisco, Mar. 10, 2021

Live Science, Everyone should wear face 'masks' in public, CDC now recommends, Apr. 3, 2020

PNAS, An evidence review of face masks against COVID-19, Jan. 26, 2021

PNAS, Identifying airborne transmission as the dominant route for the spread of COVID-19, Jun. 11, 2020

International Journal of Nursing Studies, A rapid systematic review of the efficacy of face masks and respirators against coronaviruses and other respiratory transmissible viruses for the community, healthcare workers and sick patients, Aug. 2020

Institute for Work and Health, Experimental vs. observational studies, Feb. 2016

Wiley Online Library, The use of masks and respirators to prevent transmission of influenza: a systematic review of the scientific evidence, Dec. 21, 2011

Emerging Infectious Diseases, Face Mask use and control of respiratory virus transmission in households, Feb. 2009

The Lancet, Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis, Jun. 27, 2020

Cochrane Database of Systematic Reviews, Physical interventions to interrupt or reduce the spread of respiratory viruses, Jul. 6, 2011

Johns Hopkins Bloomberg School of Public Health, Non-pharmaceutical interventions

Annals of Internal Medicine, Effectiveness of Adding a Mask Recommendation to Other Public Health Measures to Prevent SARS-CoV-2 Infection in Danish Mask Wearers, Nov. 18, 2020

Factcheck.org, Danish study doesn’t prove masks don’t work against the coronavirus, Nov. 25, 2020

PolitiFact, No, a Danish study didn’t prove wearing masks is ineffective, Dec. 7, 2020

Science of the Total Environment, Understanding transmission and intervention for the COVID-19 pandemic in the United States, 2020

Centers for Disease Control and Prevention, Trends in County-Level COVID-19 Incidence in Counties With and Without a Mask Mandate — Kansas, June 1–August 23, 2020, Nov. 27, 2020

The New England Journal of Medicine, Visualizing speech-generated oral fluid droplets with laser light scattering, May 21, 2020

Nature Medicine, Respiratory virus shedding in exhaled breath and efficacy of face masks, Apr. 3, 2020

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