Even as officials in Texas and Mississippi lifted statewide mask mandates, researchers at the Centers for Disease Control and Prevention on Friday offered fresh evidence of the importance of face coverings, reporting that mask-wearing mandates were linked to fewer infections with the coronavirus and Covid-19 deaths in counties across the United States.
Federal researchers also found that counties opening restaurants for on-premises dining — indoors or outdoors — saw a rise in daily infections about six weeks later, and an increase in Covid-19 death rates about two months later.
The study does not prove cause and effect, but the findings square with other research showing that masks prevent infection and that indoor spaces foster the spread of the virus through aerosols, tiny respiratory particles that linger in the air.
“You have decreases in cases and deaths when you wear masks, and you have increases in cases and deaths when you have in-person restaurant dining,” Dr. Rochelle P. Walensky, the director of the C.D.C., said on Friday. “And so we would advocate for policies, certainly while we’re at this plateau of a high number of cases, that would listen to that public health science.”
The findings come as city and state officials nationwide grapple with growing pressure to reopen schools and businesses amid falling rates of new cases and deaths. Officials have recently permitted limited indoor dining in New York City. On Thursday, Connecticut’s governor said the state would be ending capacity limits later this month on restaurants, gyms and offices. Masks are still required in both locales.
“The study is not surprising,” said Joseph Allen, an associate professor at Harvard’s T.H. Chan School of Public Health and director of the university’s Healthy Buildings program. “What’s surprising is that we see some states ignoring all of the evidence and opening up quickly, and removing mask mandates and opening full dining.”
Other researchers said the new study confirmed the idea that viral transmission often takes place through the air, that physical distancing may not be sufficient to halt the spread in some settings, and that masks at least partly block airborne particles.
President Biden’s health advisers have said in recent days that now is not the time to relax. As of Thursday, the seven-day average of new cases was still 62,924 a day, according to a database maintained by The New York Times.
While that figure is down 14 percent from two weeks earlier, new cases remain near the peaks reported last summer. Though fatalities have started falling, in part because of the vaccination campaigns at nursing homes, it remains routine for 2,000 deaths to be reported in a single day.
Mr. Biden on Wednesday criticized the decisions by the governors of Texas and Mississippi to lift statewide mask mandates and reopen businesses without restrictions, calling the plans “a big mistake” that reflected “Neanderthal thinking.”
The president, who has asked Americans to wear masks during his first 100 days in office, said it was critical for public officials to follow the guidance of doctors and public health leaders as the coronavirus vaccination campaign gains momentum. As of Thursday, about 54 million people had received at least one dose of a Covid-19 vaccine.
“It may seem tempting, in the face of all of this progress, to try to rush back to normalcy as if the virus is in the rearview mirror,” Andy Slavitt, a White House adviser on the pandemic, said on Friday. “It’s not.”
C.D.C. researchers examined the associations between mask mandates, indoor or outdoor restaurant dining, and coronavirus infections and deaths last year between March 1 and Dec. 31. The agency relied on county-level data from state government websites and measured daily percentage change in coronavirus cases and deaths.
Infections and deaths declined after counties mandated mask use, the agency found. Daily infections rose about six weeks after counties allowed restaurants to open for dining on the premises, and death rates followed two months later.
The report’s authors concluded that mask mandates were linked to statistically significant decreases in coronavirus cases and death rates within 20 days of implementation. On-premises dining at restaurants, indoors or outdoors, was associated with rising case and death rates 41 to 80 days after reopenings.
“State mask mandates and prohibiting on-premises dining at restaurants help limit potential exposure to SARS-CoV-2, reducing community transmission of Covid-19,” the authors wrote.
Shortly after publishing the report, the C.D.C. amended it, urging establishments that resume serving diners to follow agency guidelines for reducing transmission in restaurants.
“The message is, if restaurants are going to open for on-premise dining, it’s important to follow C.D.C. guidelines to do so safely and effectively,” said Gery P. Guy, a scientist with the C.D.C.’s Covid response team and the study’s corresponding author.
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That includes “everything from having staff stay home when they show signs of Covid or have tested positive or been in contact with someone who has Covid, and requiring masks among employees as well as customers who are not actively eating or drinking,” Dr. Guy said.
Other steps include adequate ventilation, options to eat outdoors, spacing customers six feet apart, encouraging frequent hand washing, and sanitizing of surfaces that are touched a lot, such as cash registers or pay terminals, door handles and tables.
Even if restaurants limit capacity, however, aerosolized virus may accumulate if ventilation is inadequate, Dr. Allen said.
“It doesn’t really matter if it’s a restaurant, spin class, a gym, a choir practice — if you’re indoors with no masks, low or no ventilation, we know that’s higher risk,” he said. “Respiratory aerosols build up indoors. It’s that simple. This is a real problem for restaurants.”
Linsey Marr, an expert on aerosol transmission at Virginia Tech, said Americans could not be expected to follow all the latest science, and so many simply rely on what is open or closed as an indicator of what is safe.
But indoor dining is particularly risky, she added. People typically sit in a restaurant for an hour or more and don’t wear masks while eating, leaving them vulnerable to airborne virus.
“Limiting capacity will help reduce the risk of transmission, but indoor dining is still a high-risk activity until more people are vaccinated,” she said.
Restaurant workers are particularly exposed. While they can wear masks, diners do not, reducing protection against the virus. And workers spend many hours inside with every shift, Dr. Allen said.
He recommended that restaurant workers double-mask, wearing a surgical mask covered by a cloth mask, or buy high-efficiency masks like N95s, typically reserved for health care workers, or KN95 or KF94 masks, taking steps to assure they are not counterfeit.
“Now is not the time to let our guard down and pull back on the controls when we’re so close to having a lot of people vaccinated,” Dr. Allen said.
Eileen Sullivan contributed reporting.