John Moore, a virologist at Weill Cornell Medicine in New York, said people would need to assess their own comfort in different situations, depending on the size of the gathering and the number of cases in the area.

“Would I go to a modest dinner party with vaccinated friends?” he said. “Absolutely. But walking into a bar in a poorly vaccinated state, or walking into a large gathering of people — I would be uncomfortable doing that without a mask.”

“I know people who are of my age who are very very skittish about any form of mingling,” added Dr. Moore, who said he is in his 60s. “It’s going to take a lot of adjustment, but I think it’s a good idea, and appropriate on the science.”

In a sense, the agency is asking that neighbors, colleagues and complete strangers trust one another to do the right thing, some scientists noted. Shedding masks may rekindle a national debate on vaccine passports, as verification of immunity becomes more important in unmasked settings like offices and restaurants.

Ellie Murray, an epidemiologist at the Boston University School of Public Health, said, “Basically, what that relies on, then, is individuals policing people around them, or business owners checking vaccination status in some way or just relying on some sort of honor code.”

In justifying the recommendations, agency officials pointed to several recent studies showing that vaccines are more than 90 percent effective at preventing mild and severe disease, hospitalizations and deaths from Covid-19 in real world settings.

Among them was a study of 6,710 health care workers in Israel, including 5,517 fully vaccinated workers, that found that the Pfizer-BioNTech vaccine was 97 percent effective at preventing symptomatic infections among the fully vaccinated and 86 percent effective at preventing asymptomatic infections among them.