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, and Dr. Walensky said that masks were especially crucial given the concern about new variants circulating.

“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.”