Federal health officials on Wednesday urged Americans to keep their masks on and take steps to make them fit more snugly — or even to layer a cloth covering over a surgical mask — saying that new research had shown that masks greatly reduce the spread of the coronavirus.
Recent laboratory experiments found that viral transmission could be reduced by 96.5 percent if Americans wore snug surgical masks or a cloth-and-surgical-mask combination. In announcing the findings, Dr. Rochelle P. Walensky, director of the Centers for Disease Control and Prevention, pleaded with Americans to wear a “well-fitting mask.”
“With cases, hospitalizations and deaths still very high, now is not the time to roll back mask requirements,” she said. “The bottom line is this: Masks work, and they work when they have a good fit and are worn correctly.”
Masking is now mandatory on federal property and on domestic and international transportation. Studies conducted in households in Beijing, hair salons in Missouri and aboard an aircraft carrier in Guam have proved that “any mask is better than none,” said Dr. John T. Brooks, the chief medical officer at the C.D.C. and lead author of the agency’s new research on masking.
“Wearing a mask reduces spread, and in communities that adopt mask-wearing, new infections go down,” Dr. Brooks said.
But while masks reduce the respiratory droplets and aerosols exhaled by infected wearers, and protect uninfected wearers, air leaking around the edges of a mask can reduce its effectiveness. The agency’s new laboratory experiments showed how to fix the problem.
One option is to wear a cloth mask over a surgical mask, the agency said. The alternative is to fit the surgical mask more tightly on the face by “knotting and tucking” — that is, knotting the two strands of the ear loops together where they attach to the edge of the mask, then folding and flattening the extra fabric at the mask’s edge and tucking it in for a tighter seal.
The agency’s experiments relied on three-ply surgical and cloth masks, and only one type of each mask was tested. Other combinations — like doubling up on cloth masks or wearing two surgical masks, or layering a surgical mask over a cloth mask — were not tested.
The advice arrives even as states have begun to lift measures intended to slow transmission of the virus. Roughly three dozen states have masking requirements, but on Monday, Iowa ended its mandate, joining Mississippi and North Dakota, which did so months ago.
States are rushing to restart businesses and reopen schools. Gov. Andrew Cuomo of New York, for example, announced on Wednesday that fans would be allowed to return to stadiums and arenas for sporting events and concerts, at limited capacity with mandatory testing and seating. New York City will permit indoor dining to resume on Friday, at 25 percent capacity.
Virus-related deaths, which resurged sharply in the United States in November and remain high, appear to be steadily declining. New cases and hospitalizations, too, began to drop last month.
But the C.D.C. has warned that even as cases have declined, the new variants could send infections soaring if Americans let down their guards. Cases of a more contagious virus variant first found in Britain are doubling roughly every 10 days in the United States. The C.D.C. cautioned last month that it could become the dominant variant in the nation by March.
Until the vast majority of adults are vaccinated, “we want to tamp this down,” said Dr. William Schaffner, an infectious disease specialist at Vanderbilt University. Masks are an effective, easy way to do so, avoiding another catastrophic “roller coaster,” he added.
“The fewer opportunities we give this virus to multiply, the less likely it will be that mutations occur and the less likely it will be that we get new variants,” Dr. Schaffner said.
Dr. Monica Gandhi, an infectious disease specialist at the University of California, San Francisco, is the co-author of a paper about making masks more effective that inspired the C.D.C. to conduct the new research.
“We want to do our best to tamp down transmission using all the elements: masking, distancing, hand hygiene, ventilation,” she said. “If we bring down transmission and we’re mass-vaccinating at the same time, the virus doesn’t have an opportunity to evade the vaccine.”
The C.D.C. outlined a few additional options for improving the effectiveness of masks, including using a mask-fitter — a frame contoured to the face — over a mask. Recent studies found that fitters can increase protection against virus-containing aerosols by 90 percent or more.
Surprisingly, perhaps, the agency also suggested that people consider wearing a sleeve of sheer nylon hosiery material around the neck and pulled up atop a cloth or surgical mask.
The C.D.C.’s recommendations were based in part on ideas articulated by Dr. Gandhi and Linsey Marr, an expert in aerosol transmission at Virginia Tech. The two have recommended a surgical mask covered by a tightly fitting cloth mask, or a three-layer cloth mask consisting of two outer layers of tightly woven fabric that hug the face and a middle layer of filter material, like vacuum bag material.
Both the tight fit and the filtration are important, Dr. Marr said. Even with an N95 respirator mask, the kind used by health care workers, a good fit is essential.
While a growing number of Americans say they support the wearing of masks, resistance persists among some circles and in some regions. Dr. Marr said she expected the C.D.C.’s new advice to receive some ridicule.
“I’m sure the resisters will say, ‘What’s next? Three masks? Four masks?’” Dr. Marr said. “But there is a ton of interest from people who want to know how good their masks are, and how can they make them better. People want the best possible protection.”
The C.D.C. experiments simulated the production of aerosols from a cough and estimated their absorption. While an unknotted surgical mask blocked 42 percent of the particles and a cloth mask alone blocked 44.3 percent, the combination of a cloth mask over a surgical mask blocked 92.5 percent of the coughed particles, Dr. Brooks and his colleagues found.
When both the source of the aerosols and the exposed form were fitted with either the combination of masks or the knotted-and-tucked surgical mask, the exposure to the receiver was reduced by 96.4 percent and 95.9 percent.
Neither of the two methods was perfect: knotting and tucking makes the mask’s surface smaller, and may be more appropriate for people with smaller faces, Dr. Brooks noted.
Likewise, the cloth and surgical mask combination works well, but makes the mask thicker and may make it harder for some people to breathe. The additional layers also may obstruct peripheral vision, increasing the risk of tripping or falling.
Breathability is also important, Dr. Marr said. “If you layer too many things on each other that make it hard to breathe through it’s counterproductive: It’s more likely air will find gaps to leak in through,” she said.
Dr. Brooks emphasized that masking, as Americans currently practice it, is not “insufficient.” But the new advice offers “an opportunity to take it to the next level.”
“Now we’re worried about forms of the virus that may transmit more efficiently or undercut the utility of existing diagnostics, therapeutics and vaccines,” he added. “We need to up our game to spread the slow of the virus and slow its evolution.”
Sheryl Gay Stolberg contributed reporting from Washington.