The advice from federal health officials that fully vaccinated people could drop their masks in most situations took Americans, from state officials to scientific experts, by surprise. Even the White House has been notified less than a day in advance by the Centers for Disease Control and Prevention, press secretary Jen Psaki said at a news conference on Friday.

“The CDC, the doctors and medical experts there, have determined what these guidelines will look like based on their own data and the schedule,” said Ms. Psaki. “That wasn’t a White House decision.”

For months, federal officials have been vigorously warning that wearing masks and social distancing are necessary to contain the pandemic. So what has changed?

Dr. Rochelle P. Walensky, CDC director, presented the new recommendations on Thursday, citing two recent scientific findings as key factors: Few vaccinated people become infected with the virus, and transmission appears to be even less common. and the vaccines appear to be effective against all known variants of the coronavirus.

At this point there is no doubt that the vaccines are strong. On Friday, the CDC released results from another major study showing that the vaccines manufactured by Pfizer-BioNTech and Moderna are 94 percent effective in fully vaccinated patients and 82 percent effective in partially vaccinated patients.

“The science is pretty clear on this,” said Zoë McLaren, a health policy expert at the University of Maryland, Baltimore County. There is growing evidence to suggest that vaccinated people are very unlikely to catch or transmit the virus, she noted.

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

One of the scientists’ lingering concerns was that even a vaccinated person could carry the virus – perhaps briefly, with no symptoms – and spread it to others. However, CDC research, including the new study, consistently found few infections in those who received the Pfizer-BioNTech and Moderna vaccines.

“This study, which was added to the many previous studies, was instrumental in changing the CDC’s recommendations for those fully vaccinated against Covid-19,” said Dr. Walensky in a statement on Friday.

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

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

Still, most of the data was collected on the Pfizer-BioNTech and Moderna vaccines, warned Dr. Krammer. Because the Johnson & Johnson vaccine was later approved, there are fewer studies evaluating its effectiveness.

In clinical trials, the Johnson & Johnson vaccine was 72 percent effective – less than the Pfizer and Moderna vaccines. Efficacy was measured against moderate and severe illness rather than mild illness.

“It’s a very good vaccine and I’m sure it will save many, many, many lives,” said Dr. Krammer. “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 of particular concern to scientists. While Dr. Walensky citing evidence that the mRNA vaccines like those from Pfizer and Moderna are effective against the variants circulating in the US, there is little data on variants and the Johnson & Johnson vaccine. And new variants are constantly emerging.

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

Updated

May 15, 2021 at 1:37 p.m. ET

In a statement on Friday, a CDC spokesman said: “All approved vaccines offer strong protection against serious illness, hospitalization and death. We are collecting data that our approved vaccines are effective against the variants circulating in this country. ”

Fully immunized people are unlikely to get seriously ill even if infected with the coronavirus. The risk of infection is greater for those around them – unvaccinated children and adults, or vaccinated people who are left unprotected due to illness or treatment.

CDC officials said they weighed these factors and are confident about assessing the science. And the new advice has other beneficial effects: It rewards fully vaccinated people by giving them permission to end their social isolation – and possibly encouraging others to choose to vaccinate.

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

“It is unlikely that we will see another large spike in some cases,” he added. “But will the last stretch take weeks or months is still a question.”

The difficulty with the new recommendations, he and other experts said, is less the science that underpins them than their implementation.

Executives at the state, city and county level still have the authority to require masks for people who have been vaccinated, as the CDC quickly confirmed on Thursday. Following the agency’s announcement, some states immediately lifted the mask mandates, while others said they would need more time to weigh the evidence.

In states without a mask mandate, shopkeepers, restaurant workers, school officials and workplace managers must check vaccination status.

“Without a means of checking vaccination, we have to rely on an honor system,” said Caitlin Rivers, an epidemiologist at Johns Hopkins University.

The number of cases in the country is the lowest since September, and many experts are supporting the lifting of mask mandates across much of the country. But this will be riskier in places like Michigan, where there are more cases and for people who are unprotected, including children under the age of 12 and people with weak immune systems, said Dr. Rivers.

“People who are not vaccinated should continue to wear masks indoors in public and avoid crowds,” she said.

In Nacogdoches, Texas, Dr. Ahammed Hashim that only 36 percent of the population were vaccinated and the pace seemed to have stalled. Yet only one or two in ten people in local shops wore masks.

“I think the CDC could send the wrong message that everything is fine,” said Dr. Hashim, a pulmonologist. “It would feel a lot better if we had a 60 or 70 percent vaccination.”

The CDC guidelines are aimed at fully vaccinated individuals and should only be interpreted as such, warned Dr. Sharpstein. Nationwide, only 36 percent of the population are fully vaccinated.

“What we are seeing right now is a small gap between advice that is perfectly appropriate for people who have been vaccinated and the fact that there are places where virus transmission still takes place and a lot of people who are not vaccinated. ” he said.

Individuals can make decisions based on their perception of their own risks, but state and local leaders must decide what is best for the community based on the rate of infection. “These are two different things,” said Dr. Sharpstein. “And when they get into conflict, people can make bad judgments about politics.”

The new guidelines should remind health authorities to expand their reach and investment to ensure everyone has access to vaccines, said Dr. McLaren. Parents of children under the age of 12 should continue to encourage them to wear masks around the house.

The CDC’s new policy also shifts responsibility to immunocompromised people to protect themselves from exposed and unvaccinated people.

“When we make politics, we have to balance everyone’s needs and wants,” said Dr. McLaren. “We could mask forever, but there are benefits in going back to a life that looks more normal.”

Health officials should emphasize that the situation may still change, and official recommendations on that, she added, “We really need to practice being responsive to changing situations.”