When the White House coronavirus task force first recommended mask-wearing April 3, officials emphasized that it was not about the mask-wearer. It was about others.
Now, a growing body of science experts suggest that wearing masks to prevent spreading the virus helps everyone. It is further evidence that knowledge about masks, and their benefits, continues to evolve.
The Centers for Disease Control and Prevention publicly acknowledged that for the first time, writing in a scientific bulletin posted to its website this week, "the benefit of masking is derived from the combination of source control and personal protection for the mask wearer."
John Brooks, chief medical officer for the CDC's coronavirus response, said there is an urgency to explain this clearly, because the widespread wearing of face coverings can help avoid a return to lockdowns.
The CDC published this scientific brief to fix what the agency saw as the lack of "a concise summary of the powerful scientific evidence demonstrating the benefit of masking," he said. The bulletin marks the start of the agency's renewed push to bolster public messaging as infections surge to highest-ever levels in many U.S. regions.
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Because the CDC cannot impose mandates, the agency wants the public to understand masks are "good for them," Brooks said.
The new document attempts to dangle a persuasive carrot of information before the public: A "likely complementary and possibly synergistic" relationship exists between controlling the source of infection for others and being protected yourself, the agency said. Put another way, the more people wearing masks in the community, the greater the individual benefit.
"Wearing a mask blocks you from inhaling potential virus-containing particles in the air," Brooks said. "But most of the benefit to a mask is to block particles coming out of people who don't know they are infected from exposing others."
Masks create a barrier that stops some of the droplets from flying outward when someone breathes, talks, sings or coughs. A study released last week showed that, in experimental conditions, simple fabric masks blocked about three-fourths of the particles expelled by coughing volunteers.
But it was a logical hypothesis for researchers to investigate, too, whether masks might also block incoming particles. And laboratory tests in recent months indicated that masks can filter out the types of incoming particles able to carry virus, Brooks said.
The CDC official added that personal protection for the mask-wearer is not absolute. "The real benefit is when all of us do it, that's how we bring down the viral load of covid-19 in communities," Brooks said.
In the nine months since the virus hit the United States, the CDC has come under increasing criticism that its guidance on mask protection has not been sufficiently clear because it conflated scientific data with a concern over mask availability.
During the first wave of the pandemic, surveys indicated a majority of Democrats and Republicans wore masks, though Democrats were more likely to. But a few Americans were strongly skeptical. This opposition became so pronounced in the summer that online shops sold fake "mask exemption" cards, labeled with stolen Justice Department imagery, by the hundreds.
By midsummer, Republican members of Congress were encouraging face coverings. Senate Majority Leader Mitch McConnell, R-Ky., said in late June that putting on a mask was the "single most important thing you can do" until vaccines are available.
"Overall, this seems like a win in terms of messaging that would appeal to Republicans," said Katherine White, an expert in consumer behavior at the University of British Columbia. She said this was because conservatives are strongly motivated by a personal responsibility to care for themselves.
Brooks, who oversaw the publication of the new bulletin, said the language went through lengthy vetting to make sure there was agreement on the best evidence. In this case, laboratory, epidemiological and population-based studies all showed "very substantial benefits" of mask-wearing, he said.
An evolution of scientific thinking about masks in the United States occurred amid the pandemic. Monica Gandhi, an infectious-diseases expert at the University of California at San Francisco, said science suggests face coverings "of higher thread count, more than one layer, and those that employ 'electrostatic filtration' like surgical masks" provide the best protection.
"Messaging that allows the public to know that masks protect you as well as others will be more powerful in convincing skeptics that masks are important in public spaces to slow down spread and disease from covid-19," Gandhi said.