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VERIFY: Inaccurate claims about CDC study miss purpose of masks

President Trump and others have made inaccurate claims about masks by cherry-picking one data point from a CDC report and ignoring the rest of the information.

Fox News host Tucker Carlson and President Donald Trump this week both amplified a claim that masks don’t stop the spread of COVID-19, citing a weekly report from the Centers for Disease Control and Prevention published in early September.

The president and Carlson claimed masks don’t work because a large percentage of people who contracted the coronavirus in a study wore masks.

However, that’s an incorrect interpretation of the study and the CDC has pushed back against that idea since the message began spreading.

THE QUESTION

Did a CDC study prove masks don’t work?

THE ANSWER

No. The CDC has pushed back against that claim and stated people who think masks don't work are misinterpreting both the study and the CDC’s messaging.

WHAT WE FOUND

The study referenced by Carlson and Trump is the Morbidity and Mortality Weekly Report from Sept. 11. It specifically studied the prior behaviors of COVID-19 patients, based on interviews of a small sample size of outpatients from 11 academic medical facilities.

Of the 154 COVID-19 patients interviewed, 70.6% said they “always” wore a mask in the 14 days before illness onset and another 14.4% said they “often” wore a mask during the same period. Because of this, Carlson and Trump incorrectly claimed that it showed 85% of COVID-19 patients wear masks.

The CDC publicly said on Twitter this interpretation of the study is incorrect.

A CDC spokesperson told the VERIFY team in an email, “In studies like the MMWR published in September, it is difficult to detect the effect of an exposure or intervention when it is widely deployed or used. Both cases and controls in the study had high levels of mask use. They were likely achieving sufficient protection within their social network. However, in restaurants and other places where food and beverages are consumed, masks cannot be used. These are locations where even mask users become vulnerable by dropping their guard to exposure.”

So, the CDC said that mask use was high among both the case group, that’s the COVID-19 patients, and the control group, symptomatic patients who tested negative for COVID-19. Nearly 89% of the 160 control group patients wore masks always or often, so there really wasn’t a difference in mask use by the two groups.

The CDC also noted in their emailed statement that masks cannot be used in places like restaurants. In fact, 41% of the case patients said they had gone to a restaurant in the last 14 days before they became ill, while only 28% of the control group said they had been to a restaurant. It’s the largest difference in participation in a single activity between the two groups in the entire report.

The CDC spokesperson also noted in their email, “CDC guidance on masks has clearly stated that wearing a mask is intended to protect other people in case the mask wearer is infected. At no time has CDC guidance suggested that mask[s] were intended to protect individual wearers.” That’s consistent with CDC’s messaging since they first issued guidance that the general public should wear masks.

Credit: AP
President Donald Trump wears a mask as he tours the Whirlpool Corporation facility in Clyde, Ohio, Thursday, Aug. 6, 2020. (AP Photo/Susan Walsh)

Messaging about masks everywhere on the COVID-19 section of the CDC website, from the COVID-19 FAQ page, to the general mask homepage and the page on considerations for masks, all highlight that the primary purpose of masks during the COVID-19 pandemic is to prevent the wearer from spreading the virus to others. This is because people with COVID-19 can be infectious before they experience symptoms, while they’re experiencing mild symptoms and even while they’re asymptomatic. 

That means even if someone is wearing a mask while grocery shopping, the virus can still be spread to them by someone nearby who isn’t wearing a mask. 

Medical experts have largely argued that wearing masks reduces the spread of COVID-19, but doesn't eliminate the spread entirely.

That's important to note because these claims also ignore the sample of people in this CDC report. The 154 COVID-19 patients and the 160 control patients came from 11 medical facilities spread among just 10 states. That sample isn’t necessarily representative of the nationwide populace, and the study itself notes that as one of its limitations.

If patients from a hospital in an area where mask usage is high are interviewed, it’s more likely most patients will have worn masks before getting COVID-19. But even if every COVID-19 patient exiting a hospital in that area has worn a mask, that doesn’t mean masks are failing if the area is still seeing less spread of COVID-19 than the national average.

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The CDC study doesn’t break down the data of general mask usage for each hospital’s wider area and the overall rate of COVID-19 spread in those areas. The goal of the study wasn’t to measure the effectiveness of mask use, but instead to measure common points of exposure to COVID-19.

But a number of studies have suggested masks are effective in curbing the spread of COVID-19. 

“Growing evidence increasingly shows that wearing masks in community settings reduces transmission among individuals in that community," the CDC spokesperson said. "There are laboratory studies, animal studies, community and epidemiological studies, as well as policy studies that show masking reduces transmission in communities conferred largely by source control (blocking exhaled respiratory droplets) and as personal protection (prevents inhalation of infected respiratory particles).”

So, the bottom line is that this one study doesn’t contradict those many other studies supporting the use of masks.

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