But studies of influenza pandemics have shown that when high-grade N95 masks are not available, surgical masks do protect people a bit more than not wearing masks at all. And when masks are combined with hand hygiene, they help reduce the transmission of infections.

When researchers conducted systematic review of a variety of interventions used during the SARS outbreak in 2003, they found that washing hands more than 10 times daily was 55 percent effective in stopping virus transmission, while wearing a mask was actually more effective — at about 68 percent. Wearing gloves offered about the same amount of protection as frequent hand-washing, and combining all measures — hand-washing, masks, gloves and a protective gown — increased the intervention effectiveness to 91 percent.

With the current coronavirus, researchers are also finding that there are more asymptomatic cases than were known early on in the pandemic. Classified data from the Chinese government that was reported in the South China Morning Post indicated that up to a third of all people who tested positive for the coronavirus could have been silent carriers. Widespread testing on the Diamond Princess showed that half of the positive cases on board the cruise ship had no symptoms. And officials in Iceland, who have tested a high proportion of citizens in the country, have found similarly high percentages of asymptomatic infection.

A new report from the C.D.C., published Friday, also suggests that several residents of a nursing facility in King County, Washington, either did not have any symptoms or developed very mild symptoms only after they had been confirmed to have a coronavirus infection.

Since identifying and isolating people with infections plays a major role in breaking the chain of transmissions, people who do not have any symptoms or do not develop symptoms until later may continue to inadvertently spread the disease.

“It’s still hard to tell what percentage of people are truly asymptomatic because many go on to develop symptoms a few days later,” said Dr. Neil Fishman, the chief medical officer of the Hospital of the University of Pennsylvania. “What we do know is that individuals can shed virus about 48 hours before they develop symptoms and masking can prevent transmission from those individuals.”

Wearing a mask can also reduce the likelihood that people will touch their face, which is another mode of transmission of the virus from contaminated surfaces to unsuspecting individuals, Dr. Fishman said.