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Posts misrepresent CDC study on mask mandates

March 11, 2021 GMT

CLAIM: A new study from the U.S. Centers for Disease Control and Prevention found that mask mandates have a negligible impact on coronavirus numbers.

AP’S ASSESSMENT: False. The CDC’s Morbidity and Mortality Weekly Report released March 5 found that state-issued mask mandates were associated with statistically significant decreases in county-level daily COVID-19 case and death growth rates, according to CDC scientist Gery Guy Jr., lead author of the study.


THE FACTS: Social media users and conservative websites are sharing a recent study from the CDC with false claims that it shows mask mandates don’t stem the spread of the coronavirus. 

“Here is your proof from the CDC itself that masks don’t work,” one Facebook user wrote with the link to the study. “Study released on Friday says masks resulted in only 1-2% spread reduction over 100 days. Basically useless.”

The conservative television network One America News reported that the study showed masks have “negligible impact” and mask mandates “do not make any statistical difference.”

Those claims distort the study’s findings, according to Guy. While the changes in daily COVID-19 case and death growth rates in the report may seem small, they have significant impact, he said. 

“Our study found that mask mandates were associated with statistically significant decreases in county-level daily COVID-19 case and death growth rates,” Guy wrote in an email to The Associated Press. “In addition, allowing on-premises restaurant dining was associated with increases in county-level case and death growth rates.”

The CDC researchers looked at U.S. counties placed under state-issued mask mandates and at counties that allowed restaurant dining — both indoors and at tables outside. The study looked at data from March through December of last year.

The scientists found that within the first 20 days of implementation, mask mandates were associated with a 0.5 percentage point decrease in daily COVID-19 case growth rates. As time went on, reductions in daily growth rates for new cases and deaths increased to nearly 2 percentage points.


Those changes in daily growth rates may sound small, but their magnitude adds up quickly, Guy said.

“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 in a previous report.

Several social media posts also claimed that the results of the study were “inside the margin for statistical error” and therefore not significant.

That’s false, according to Guy. He said those social media users may have misunderstood a sentence in the study that read, “Daily case and death growth rates before implementation of mask mandates were not statistically different from the reference period.”

The sentence refers to the period before mask mandates were implemented, not after. Guy said the research team examined this period to help rule out that there was not already a trend in place before mask mandates began.


This is part of The Associated Press’ ongoing effort to fact-check misinformation that is shared widely online, including work with Facebook to identify and reduce the circulation of false stories on the platform. 

Here’s more information on Facebook’s fact-checking program: