The data, which compared mask mandates and closure policies with infection rates from all 3,142 U.S. counties in 2020, was published as indoor dining resumes in many parts of the country.

Throughout the pandemic, experts have agreed that wearing masks slows the spread of COVID-19 and that dining in restaurants increases the number of cases. Still, experts (and most people) want to see the science behind these assertions. On Friday, the Centers for Disease Control and Prevention released exactly that data—showing that from March 1 to the end of 2020, counties with mask mandates saw their cases and death rates decrease while counties that allowed on-site dining saw their cases and death rates grow.

The study—with a dozen authors from the CDC COVID-19 Response Team, the CDC Public Health Law Program, and the University of Nevada, Las Vegas—looked at "county-level data on state-issued mask mandates and restaurant closures" across all 3,142 U.S. counties and then analyzed the case and death growth rates in 20-day increments from 60 days before until 100 days after the corresponding change in policy.

Vintage padded bar stools in an American diner restaurant.
Credit: Oleksandra Korobova/Getty Images

"Mask mandates were associated with statistically significant decreases in county-level daily COVID-19 case and death growth rates within 20 days of implementation," the report stated. "Allowing on-premises restaurant dining was associated with increases in county-level case and death growth rates within 41–80 days after reopening." As a result, the CDC essentially reiterated their existing policy: "Mask mandates and restricting any on-premises dining at restaurants can help limit community transmission of COVID-19 and reduce case and death growth rates."

Worth noting is that, based on how the data was collected, "the analysis did not differentiate between indoor and outdoor dining." However, it stands to reason that if any dining leads to an increase in case levels, indoor dining almost certainly carries at least an equal risk to outdoor dining. The report also points out that "the models did not control for other policies that might affect case and death rates."

Maybe not coincidentally, the study—which is marked as an "early release"—arrived just three days after Texas Governor Greg Abbot made a headline-grabbing announcement that the country's second-most populous state would be lifting its mask mandate and allowing all restaurants to open up to 100 percent capacity if they choose as of this Wednesday. Mississippi made a similar announcement the same day.

Those decisions went directly against the advice CDC Director Rochelle Walensky gave last Monday, warning that a fourth wave of COVID-19 was possible if restrictions were lifted too early. "The next month or two is really pivotal in terms of how this pandemic goes," she said. By Wednesday, President Biden chimed in, calling Texas's policy shift "a big mistake" based on "Neanderthal thinking."

And though masks will now be optional in Texas, in response to Abbot's announcement, the Texas Restaurant Association reiterated that masks should still be part of the plan. "In keeping with the public health guidance and best practices, our updated Texas Restaurant Promise includes a face covering requirement for employees, and encourages guests to wear a face covering when they are not seated at their table," the group wrote.