John Moore, a virologist at Weill Cornell Medicine in New York, said people would need to assess their own comfort levels in different situations, depending on the size of the congregation and the number of cases in the area.
"Would I go to a humble dinner party with vaccinated friends?" he said. "Absolutely. But going to a bar or a large crowd of people with a badly vaccinated condition – I would be uncomfortable doing that without a mask."
"I know people my age who are very, very upset about any kind of intermingling," added Dr. Moore added, who said he was in his 60s. "It's going to take a lot of adjustment, but I think it's a good idea and science-appropriate."
In a way, the agency is asking neighbors, colleagues, and total strangers to trust each other in order to do the right thing, some scientists noted. Throwing off masks can rekindle a national vaccination passport debate as immunity verification becomes increasingly important in unmasked settings such as offices and restaurants.
Ellie Murray, an epidemiologist at Boston University School of Public Health, said, "Basically, it depends on people monitoring people around them, or business owners checking vaccination status in some way, or just relying on some kind of honor code."
To justify the recommendations, agency officials pointed to several recent studies showing vaccines are more than 90 percent effective at preventing in-practice mild and serious illness, hospitalization and deaths from Covid-19.
Among them was a study of 6,710 health care workers in Israel, including 5,517 fully vaccinated workers, which found the Pfizer BioNTech vaccine 97 percent in preventing symptomatic infections among the fully vaccinated and 86 percent in preventing asymptomatic ones Infections was effective for them.