It has become the battle cry of public health officials around the world: "Wear a mask to slow the spread." Friday, the U.S. Centers for Disease Control and Prevention released new evidence supporting this advice.

Researchers analyzed coronavirus infection rates in Kansas following a statewide mask mandate. They found that counties that chose to enforce the mandate saw their cases decrease. Counties that chose to opt out saw their cases continue to rise.

"This adds to the growing body of evidence that says large, widespread masking helps to slow the spread of COVID," says Dr. Aaron Carroll, a professor at Indiana University School of Medicine.

Carroll cautions that this was not a randomized, controlled study and there could have been other factors at play (such as more physical distancing in social situations and fewer large gatherings) in the counties that were enforcing masks.

Still, as the study notes, the findings were consistent with declines in coronavirus cases observed in 15 states and the District of Columbia where masks were mandated, compared with states that didn't require the face coverings.

The Kansas mask requirement went into effect on July 3, when coronavirus cases were rising across the state. But 81 counties opted out of the mandate, as permitted by state law. The other 24 counties — which account for the majority of the state's population — chose to require that masks be worn in public places.

The CDC and the Kansas Department of Health and Environment analyzed trends in county-level cases before the mandate went into effect and two months afterward. Though rates were considerably higher in the 24 counties that required masks, over the two-month study period they brought the growth of cases under control and even reduced them. The counties that didn't require masks continued to see their cases increase.

On average, the counties that required masks saw a 6% reduction in cases (calculated as a seven-day rolling average of new daily cases per capita). In contrast, the counties that opted out saw a 100% increase.

Along with other mitigation strategies including physical distancing and hand washing, "the decrease in cases among mandated counties and the continued increase in cases in nonmandated counties adds to the evidence supporting the importance of wearing masks," the CDC says.

The coronavirus is a respiratory virus. It spreads from person to person, primarily via respiratory droplets expelled when we are in close proximity to others. These droplets can hang in the air — especially indoors, in poorly ventilated spaces. So, blocking the dispersion of such droplets with a mask is a good strategy to cut down on transmission. The CDC recently updated its guidance to clarify that masks protect the person wearing the mask as well as other people.

"You wear masks because the evidence suggests it not only protects you from acquiring the infection, but it protects others around you," says Jeffrey Shaman, an infectious disease expert at Columbia University.

He says it's been "incredibly frustrating" to see the use of masks become a partisan wedge issue instead of being seen by everyone as a sensible, public health strategy. "You do it to protect your loved ones, to protect your neighbors. You do it for the good of the country."

Shaman acknowledges that "we're all exhausted by this virus. But the reality is the virus doesn't care. All it looks for is the opportunity to move from person to person," he says.

And a mask is a harmless, cheap way of slowing it down.

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