Advice from federal health officials that fully vaccinated people could drop their masks in most settings came as a surprise to Americans, from state officials to scientific experts. Even the White House got less than a day’s notice from the Centers for Disease Control and Prevention, the press secretary, Jen Psaki, said at a news briefing today.

“The CDC, the doctors and medical experts there, are the ones who determined what this guidance would be based on their own data, and what the timeline would be,” Psaki said. “That was not a decision directed by or made by the White House.”

For months, federal officials have vigorously warned that wearing masks and social distancing were necessary to contain the pandemic. So what changed?

Introducing the new recommendations on Thursday, Dr. Rochelle Walensky, the CDC director, cited two recent scientific findings as significant factors: Few vaccinated people become infected with the virus, and transmission seems rarer still; and the vaccines appear to be effective against all known variants of the coronavirus.

There is no doubt at this point that the vaccines are powerful. Today, the CDC released results from another large study showing that the vaccines made by Pfizer-BioNTech and Moderna are 94% effective in preventing symptomatic illness in those who were fully vaccinated, and 82% effective even in those only partly vaccinated.

“The science is quite clear on this,” said Zoë McLaren, a health policy expert at the University of Maryland, Baltimore County. Mounting evidence indicates that people who are vaccinated are highly unlikely to catch or transmit the virus, she noted.

The risk “is definitely not zero, but it’s clear that it’s very low,” she said.

One of the lingering concerns among scientists had been that even a vaccinated person might carry the virus — perhaps briefly, without symptoms — and spread it to others. But CDC research, including the new study, has consistently found few infections among those who received the Pfizer-BioNTech and Moderna vaccines.

“This study, added to the many studies that preceded it, was pivotal to CDC changing its recommendations for those who are fully vaccinated against COVID-19,” Walensky said in a statement today.

Other recent studies confirm that people who are infected after vaccination carry too little virus to infect others, said Florian Krammer, a virologist at the Icahn School of Medicine at Mount Sinai.

“It’s really hard to even sequence the virus sometimes because there’s very little virus, and it’s there for a short period of time,” he said.

Still, most of the data has been gathered on the Pfizer-BioNTech and Moderna vaccines, Krammer cautioned. Because Johnson & Johnson’s vaccine was authorized later, there are fewer studies assessing its effectiveness.

In clinical trials, the Johnson & Johnson vaccine had 72% efficacy — lower than the figure for the Pfizer and Moderna vaccines. And effectiveness was measured in terms of moderate and severe disease, rather than mild disease.

“It’s a very good vaccine, and I’m sure it will save many, many, many lives,” Krammer said. “But we need more data on how well the J&J vaccine prevents infection, and how well it prevents transmission.”

Variants of the virus have been a particular worry for scientists. While Walensky cited evidence showing that the mRNA vaccines like those from Pfizer and Moderna are effective against the variants circulating in the United States, there is little data about variants and the Johnson & Johnson vaccine. And new variants are emerging constantly.

“I’m not at all saying that this is now a big problem,” Krammer said. But before lifting the masking requirements, “I might have waited a little bit longer to look at the numbers.”

In a statement today, a CDC spokesman said, “All of the authorized vaccines provide strong protection against serious illness, hospitalization, and death, and we are accumulating data that our authorized vaccines are effective against the variants that are circulating in this country.”

Fully immunized people are unlikely to get seriously ill, even if they are infected with the coronavirus. The risk of infection is greater for the people around them — unvaccinated children and adults, or vaccinated people who remain unprotected because of a medical condition or treatment.

CDC officials said they weighed those factors and were confident in their assessment of the science. And the new advice has other salutary effects, rewarding fully immunized people by giving them permission to end their social isolation — and perhaps incentivizing others to opt for vaccination.

The new advice “signals that we really are on the final stretch here, and I think that’s a very good thing for people,” said Dr. Joshua Sharfstein, the vice dean for public health practice and community engagement at Johns Hopkins University Bloomberg School of Health.

“It’s unlikely that we’re going to have another huge surge in cases,” he added. “But will the final stretch last for weeks or months is still a question.”

The difficulty with the new recommendations, he and other experts said, is not so much the science underpinning them as their implementation.

Leaders at the state, city and county levels still have the authority to require masks even for vaccinated people, as the CDC was quick to acknowledge on Thursday. After the agency’s announcement, some states instantly lifted mask mandates, while others said they would need more time to weigh the evidence.

But in states without mask mandates, the onus of checking vaccination status will fall on shopkeepers, restaurant workers, school officials and workplace managers.

In Nacogdoches, Texas, Dr. Ahammed Hashim fretted that only 36% of the population was immunized and the pace seemed to have stalled. And yet only one or two people in 10 in the local shops wore masks.

“I think the CDC might send a wrong message saying that everything’s OK,” said Hashim, a pulmonologist. “It would feel much better if we had a 60 or 70% vaccination.”

The CDC’s guidance is intended for fully vaccinated individuals, and should only be interpreted as such, Sharfstein cautioned. Nationwide, only 36% of the population is fully vaccinated.

“What we’re just seeing is a little bit of the distance between advice that is entirely appropriate for people who are vaccinated, and the reality that there are places that still are seeing viral transmission and a lot of people who aren’t vaccinated,” he said.

The CDC’s new policy shifts the onus onto the immunocompromised as well, to protect themselves from unmasked and unvaccinated people.

“When we make policy, we need to balance the needs and desires of everyone,” McLaren said. “We could keep masking forever, but there are benefits to getting back to a life that looks more normal.”

Health officials should emphasize that the situation may yet change, and official recommendations with it, she added: “We really need to practice being good at responding to changing situations.”