The agency also offered an economic argument, saying that increasing the proportion of people who wear masks by 15 percent could prevent the need for lockdowns and cut associated losses of up to $1 trillion, or about 5 percent of gross domestic product.
The new document listed several studies that “have confirmed the benefit of universal masking,” as well as some observational studies that have given evidence of its usefulness, including an example of two masked hair stylists who had been experiencing symptoms but did not transmit the virus to any of their 67 masked clients who were later contacted. The document also referred to a study of 124 Beijing households in which mask use significantly cut transmission of the virus, and an outbreak aboard the U.S.S. Theodore Roosevelt in which face coverings appeared to have reduced risk of infection by 70 percent.
Experts said they were thrilled to see the change in the C.D.C.’s stance, and particularly the emphasis on face coverings that protect wearers.