Trump repeats inaccurate claim about masks, citing CDC study.

President Donald Trump is again questioning the effectiveness of masks in protecting against catching the coronavirus. On Thursday, he repeated an incorrect claim about masks, citing a recent report from the Centers for Disease Control and Prevention.

“Then you see CDC comes out with a statement that 85 percent of the people wearing masks catch it?" he told Fox Business anchor Stuart Varney on Thursday morning. "What's that all about?"

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Trump was referring to a Sept. 10 report from the CDC that found that dining out raised the risk of infection more than other social activities. The report has gone viral this week on social media, with the claims that the agency’s mask guidance isn’t preventing people from getting sick. On Wednesday afternoon, the CDC tweeted that “the interpretation that more mask-wearers are getting infected compared to non-mask wearers is incorrect.”

The CDC report looked at 314 people who had Covid-19 symptoms and were subsequently tested for the virus; about half tested positive. In a phone survey, participants were asked about their social activities for the two weeks prior to testing positive, such as going to church, gyms or stores. The researchers found that among those who tested positive for the coronavirus, 85 percent had reported wearing masks always or often. The study also found that for those in the group who had tested negative, 89 percent had reported wearing masks with the same frequency.

While Trump had the number right, the interpretation of the data is inaccurate, according to the CDC's report.

Dr. Todd Rice, lead author of the study and associate professor of medicine at Vanderbilt University Medical Center in Nashville, told NBC News that the research is actually “mask neutral” and that the data wasn’t designed to argue for or against mask wearing.

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A key problem with the “masks don’t stop the spread” claim: The data was self-reported through phone surveys. In other words, people told the interviewers about their mask use; it wasn’t observed through video monitoring.

“You can imagine that there may be some inaccuracies in the reporting of it,” Rice said.

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While it’s correct to say that a large portion of the participants who tested positive reported wearing a mask, the same was true for those who tested negative, Rice said. The only conclusion that can be drawn is that the majority of the people in the study reported using cloth face coverings or mask types “often” or “always” when in public.

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In fact, the biggest risk factor associated with Covid-19 was dining out, the study found. Participants who tested positive were twice as likely to have reported dining at a restaurant, where mask wearing becomes difficult.

“Even for those who report always wearing a mask, it is important to consider that there are community activities where masks cannot be worn," a CDC spokesperson said in a statement. "People who attend restaurants cannot wear masks effectively while eating and drinking, whereas customers can always wear masks when shopping or during other indoor activities.”

The CDC guidance on masks says wearing a mask is intended to protect other people in case the mask wearer is infected. “At no time has CDC guidance suggested that masks were intended to protect the wearers,” the agency spokesperson said in the statement.

Masks shouldn’t be thought of as the only way to prevent the spread of the coronavirus.

"Masks aren't perfect. They help, but they're not a guarantee that you're not going to get Covid if you wear a mask,” said Rice. “I don't think masks give you the ability to be careless in other aspects of trying not to catch this.”

It’s also what Dr. Anthony Fauci has repeated for months.

“There are some things that should be universally practiced, and that is the universal wearing of masks, avoiding crowds, keeping a distance, doing things outdoors more than indoors and washing our hands frequently," Fauci, the director of the National Institute of Allergy and Infectious Diseases, told NBC News’ Kate Snow recently. "That doesn't matter who you are. That's what you should be doing."

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