Posts Distort CDC Study Supporting Mask Mandates to Reduce COVID-19


SciCheck Digest

A study from the Centers for Disease Control and Prevention found that state-issued mask mandates were associated with significant decreases in daily COVID-19 case and death growth rates. Yet some conservative outlets and social media users falsely claim the study shows mask mandates have a negligible impact on COVID-19 outcomes.

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The growth rates of daily COVID-19 cases and deaths slowed significantly in counties where states required masks, according to a study that analyzed the impacts of last year’s state-issued mask mandates across the country.

The findings support the premise that mask requirements help limit the potential exposure to the SARS-CoV-2 virus, which therefore reduces the spread of the disease, COVID-19. But conservative outlets and social media users turned those findings around to say the CDC found face masks have a “negligible impact” on COVID-19 outcomes and that mask mandates “made no statistical difference.”

A story published by One America News Network, a conservative cable channel, on March 7, falsely claimed the CDC “has admitted face masks do little to prevent the spread” of the virus and that the numbers in the study “didn’t exceed statistical margins of error.” The story has been seen by over 230,000 users on Facebook.

A CDC spokesperson told us that’s not correct. 

“The data we now have conclusively show that widespread use of masks is a very effective way to reduce the spread of COVID-19,” said CDC’s Jasmine Reed in an email. “The CDC study referenced noted that mask mandates were associated with statistically significant decreases in county-level daily COVID-19 case and death growth rates.”  

What the Study Found

The new study cites and builds upon previous studies that have confirmed the effectiveness of mask mandates and restaurant closures in reducing COVID-19 outcomes. In this case, CDC scientists examined the association of state-issued mask mandates and on-premises restaurant dining with COVID-19 cases and deaths from March 1 to Dec. 31, 2020. To do so, it looked at data from 2,313 U.S. counties (73.6% of a total 3,142) where mask mandates were implemented and from the 3,076 (97.9%) U.S. counties that allowed restaurants to reopen, and compared that data with county-level changes in COVID-19 case and death growth rates. 

The CDC says the study controlled for some variables such as restaurant closures in the mask mandates models and mask mandates in the restaurant reopening models, as well as bar closures, stay-at-home orders, and bans of gatherings. But it did not control for other policies that could affect case and death rates, such as closure of other types of businesses and physical distancing recommendations. It didn’t account for the level of compliance and enforcement of the policies, either, and it did not make a difference between dining conditions, such as indoor or outdoor, in restaurants. 

The study found that mask mandates were associated with a 0.5 percentage point decrease in daily COVID-19 case growth rates within the first 20 days. The reductions in growth rates increased with time, reaching 1.1, 1.5, 1.7 and 1.8 percentage points within 21 to 40, 41 to 60, 61 to 80, and 81 to 100 days, respectively, after implementation.

The study also found reopening restaurants was associated with a statistically significant increase in daily COVID-19 case and death growth rates 41 days after implementation, with 2.2 and 3.0 percentage point increases in the death growth rate 61 to 80 and 81 to 100 days after restrictions were lifted. 

“What they found was that one or the other, or both, all of them, were associated with a reduction in incidence rates,” Dr. Sten H. Vermund, dean of the Yale School of Public Health, told us in a phone interview. 

Yet, shortly after the study was first published in the Morbidity and Mortality Weekly Report online on March 5, the information got distorted.

How Growth Rate Finding Was Distorted

On March 6, Fox News’ Laura Ingraham tweeted that the study found mask mandates and restaurant restrictions “have a small impact” on COVID-19 cases and deaths. Later that day, an unreliable Twitter account called Breaking911 created a tweet thread about the new study, falsely claiming that it found “mask mandates lower COVID cases by around 1.5% over a two month period.” 

But as we already explained, the study is not showing a decrease in the number of cases and deaths but a decrease in the growth rate of the daily cases and deaths. 

“Because the growth rate is exponential,” the impact compounds, José Luis Jiménez, an analytical and atmospheric chemist at the University of Colorado Boulder who was not involved in the study, told us. “So, it’s kind of like your mortgage — you change your interest a little bit, and then you save a ton of money. Because the cases compound.”

The authors of the study did not respond to an interview request from But the CDC’s Gery P. Guy Jr., the lead author, told the Associated Press that although those changes in daily growth rates may sound small, they quickly add up. 

“Each day that growth rate is going down, the cumulative effect in terms of cases and deaths adds up to be quite substantial,” Guy told the AP.

For example, if cases were growing at 5% per day and that growth rate dropped to 3.5% per day, the total number of cases could shrink substantially, particularly over time.

Vermund told since the results show changes in the daily growth rate they are “much more impressive” than they may appear. 

“This is actually quite a large effect, the opposite of what the scurrilous journalist said,” Vermund told us, referring to the OAN story.

Neysa Ernst, nurse manager at Johns Hopkins Medicine’s biocontainment unit, told us the study results are not only statistically significant, but also clinically significant. She’s been on the front lines of caring for COVID-19 patients and says every percentage point change means a lot for units like hers and overwhelmed emergency departments across the country. 

“Every percentage point has someone’s family member attached to that,” she said in a phone interview.

Other Distortions of the Study

OAN News, and other posts published during the last two weeks, also falsely claimed that the results of the study were “inside the margin for statistical error” or that “mask mandates made no statistical difference.” They misunderstood a line of the study that says “daily case and death growth rates before implementation of mask mandates were not statistically different from the reference period.”

But the “reference period” is 20 days before implementation of mask mandates.

The cherry-picked quote from the study merely says there wasn’t a significant difference in the daily case and death growth rates during three time periods before implementation, not after. As Guy explained to the AP, the study looked at growth rates before implementation to make sure there wasn’t already a trend happening before the mask mandates were put into place. 

The study did find “statistically significant decreases” after implementation of the mandates.

Vermund said these so-called pre-post analyses, which look at trends before and after you change something, are a very common way of looking at whether an intervention in a community did benefit or harm. The problem with them, he said, is that there may be other factors in the community that also change at the same time. But he said that’s less of a problem when you are looking at short timeframe, like this study does.

The OAN story, with the headline “CDC: Face Masks Don’t Prevent COVID-19, Study Finds Masks Have Negligible Impact On Coronavirus Numbers,” misinterprets the study even further by claiming the growth rate reductions apply to mask efficacy.

“The masks were 0.5 percent effective in the first 20 days of the mandates and less than 2 percent effective after 100 days,” it says. 

But the CDC spokesperson said “the study did not examine the effectiveness of masks.”

Vermund said the lack of the actual number of cases reduced or lives saved by mask mandates makes the CDC study a little difficult to understand for the public. Still, he said, the OAN story was “completely misrepresentative.”

“They don’t understand anything about epidemiology,” he said. “The fact that you see a significant slowing of the growth rate in such a minuscule length of time — that’s not negligible.”

Jiménez, who is an expert in aerosols and has been working since the beginning of the pandemic to demonstrate that the virus travels through the air, told us the results of the study are significant and important because it looks at the general public, instead of results in a laboratory.

The results would have been even stronger, he said, if people had used masks in a more effective way early on. But the science of face masks has slowly evolved, and the study analyzed data from last year when most of the population was not using masks properly. Still, he said, the results are robust.

“Masks are essential,” Jiménez said. “And how could they not be for a virus that we breathe in.”

Vermund agrees. He said there’s no question about the efficacy of masks in reducing COVID-19, based on what he called overwhelming evidence from other studies. What’s interesting about this study, he said, is to see that in a real-world setting. 

“This was just sort of a real-world assessment that in real world use, the way people use or misuse masks, is it really worth it? And the conclusion is that it’s definitely worth it because you reduce the speed of growth of the epidemic, you start to blunt the magnitude of the epidemic,” he said. 

The CDC recognized the study’s limitations; nonetheless, it concludes that policies that require universal mask use and prohibit dining at restaurants are important tools, along with “other public health strategies,” to reduce the spread of COVID-19. 

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Guy Jr., Gery P., et al. “Association of State-Issued Mask Mandates and Allowing On-Premises Restaurant Dining with County-Level COVID-19 Case and Death Growth Rates — United States, March 1–December 31, 2020.”  Morbidity and Mortality Weekly Report 2021; 70(10);350–354. 12 Mar 2021. 

Courtemanche, Charles, et al. “Strong social distancing measures in the United States reduced the COVID-19 growth rate.” Health Affairs 2020;39:1237–46. 14 May 2020. 

Joo, Heesoo, et al. “Decline in COVID-19 hospitalization growth rates associated with statewide mask mandates—10 states, March–October 2020.” Morbidity and Mortality Weekly Report 2021; 70:212–6. 12 Feb 2021.

Kaufman, Brystana G., et al. “Comparing associations of state reopening strategies with COVID-19 burden. Journal of General Internal Medicine 2020; 35:3627–34. 6 Oct 2020. 

Lyu, Wei, and George L. Wehby. “Community use of face masks and COVID-19: evidence from a natural experiment of state mandates in the US.” Health Affairs 2020; 39:1419–25. 16 Jun 2020. 

SciCheck Video: How viral deceptions spread.” 11 Mar 2021. 

Ingraham, Laura. (@IngrahamAngle). “CDC study: Mask mandates and restaurant restrictions have small impact on coronavirus cases and deaths.” Twitter. 6 Mar 2021.

Hogberg, David. “Mask mandates and restaurant restrictions have small impact on coronavirus cases and deaths: CDC.” 5 Mar 2021.

Breaking911. (@Breaking911). “NEW CDC REPORT: Mask mandates lower COVID cases by around 1.5% over a two month period (1/5).” 6 Mar 2021.

Vermund, Sten H. Pediatrician and infectious disease epidemiologist, dean of the Yale School of Public Health. Phone interview with 18 Mar 2021. 

Jiménez, José Luis. Analytical and atmospheric chemist at the University of Colorado Boulder. Phone interview with 3 Mar 2021. 

Ernst, Neysa. Nurse manager at Johns Hopkins Medicine’s biocontainment unit. Phone interview with 13 Mar 2021. 

Breaking911. (@Breaking911). “Mask mandates decreased death rates by around 1% over a one month period, according to the study (2/5).” 6 Mar 2021.

CDC: Face Masks Don’t Prevent COVID-19, Study Finds Masks Have Negligible Impact On Coronavirus Numbers.” One America News Network. Updated 7 Mar 2021.

Berge, Clint. “New CDC report shows mask-wearing, opening restaurants impact on COVID-19 cases.” 8 Mar 2021.

Morawska, Lidia, and Donald K Milton. “It Is Time to Address Airborne Transmission of Coronavirus Disease 2019 (COVID-19).” Clinical Infectious Diseases. Vol 21, Issue 9. 1 Nov 2020.

McDonald, Jessica. “The Evolving Science of Face Masks and COVID-19.” 2 Mar 2021.

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One response to “Posts Distort CDC Study Supporting Mask Mandates to Reduce COVID-19”

  1. Frank

    It looks like you’ve misspelled the word “Loder” on your website. I thought you would like to know :). Silly mistakes can ruin your site’s credibility. I’ve used a tool called in the past to keep mistakes off of my website.


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