The C.D.C. says tight-fit masks or double masking with cloth and surgical masks increases protection.

Wearing a mask — any mask — reduces the risk of infection with the coronavirus, but wearing a more tightly fitted surgical mask, or layering a cloth mask atop a surgical mask, can vastly increase protections to the wearer and others, the Centers for Disease Control and Prevention reported on Wednesday.

New research by the agency shows that transmission of the virus can be reduced by up to 96.5 percent if both an infected individual and an uninfected individual wear tightly fitted surgical masks or a cloth-and-surgical-mask combination.

Dr. Rochelle P. Walensky, director of the C.D.C., announced the findings during Wednesday’s White House coronavirus briefing, and coupled them with a plea for Americans to wear “a well-fitting mask” that has two or more layers. President Biden has challenged Americans to wear masks for the first 100 days of his presidency.

“With cases hospitalizations and deaths still very high, now is not the time to roll back mask requirements,” she said, adding, “The bottom line is this: masks work and they work when they have a good fit and are worn correctly.”

Virus-related deaths, which resurged sharply in the United States in November and still remain high, appear to be in a steady decline; new virus cases and hospitalizations began to drop last month. But researchers warn that a more contagious virus variant first found in Britain is 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.

As of Feb. 1, 14 states and the District of Columbia had implemented universal masking mandates; masking is now mandatory on federal property and on domestic and international transportation. But while masks are known to both reduce respiratory droplets and aerosols exhaled by infected wearers and to protect the uninfected wearer, their effectiveness varies widely because of air leaking around the edges of the mask.

“Any mask is better than none,” said Dr. John Brooks, lead author of the new C.D.C. study. “There are substantial and compelling data that wearing a mask reduces spread, and in communities that adopt mask wearing, new infections go down.”

But, he added, the new research shows how to enhance the protection. The agency’s new laboratory experiments are based on the ideas put forth by Linsey Marr, an expert in aerosol transmission at Virginia Tech, and Dr. Monica Gandhi, who studies infectious diseases at the University of California, San Francisco.

One option for reducing transmission 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.

Dr. Brooks cautioned that the new study was based on laboratory experiments, and it’s unclear how these masking recommendations will perform in the real world (the experiments used three-ply surgical and cloth masks).“But it’s very clear evidence that the more of us who wear masks and the better the mask fits, the more each of us benefit individually.”

Other effective options that improve the fit include using a mask-fitter — a frame contoured to the face — over a mask, or wearing a sleeve of sheer nylon hosiery material around the neck and pulled up over a cloth or surgical mask, the C.D.C. said.

Even as vaccines are being slowly rolled out across the country, the emergence of the new variants, which may respond differently to treatments or dodge the immune system to some degree, has prompted public health officials to emphasize that Americans should continue to take protective measures like masking.