Federal health officials’ decision Thursday to rescind almost all masking and distancing recommendations for fully vaccinated Americans created as much confusion as it did celebration, sending states, businesses and individuals scrambling to figure out what rules, if any, are still appropriate and when.

Many, including President Biden, hailed the relaxation of restrictions by the Centers for Disease Control and Prevention as a milestone in the nation’s return to normal. But with a majority of Americans unvaccinated, others questioned the sudden and blanket recommendation, worrying that the onus is now heavier on state and local governments, businesses and individuals to determine whether precautions are necessary.

More than a dozen physicians interviewed Friday expressed concern that the decision was premature, coming only days after regulators cleared a vaccine for 12-to-15-year-olds and while so many are still unprotected. They feared the guidelines could undercut two of the simplest and most effective tools — masks and physical distancing — for stopping the spread of a virus still infecting about 35,000 people in the United States every day.

As drug companies expand coronavirus vaccine access to children, officials are working to persuade people who are skeptical or unwilling to get vaccinated. (Luis Velarde, Jonathan Baran, Erin Patrick O'Connor/The Washington Post)

“The guidance shifts all the burden onto individuals to be ‘on their honor’ and choose the appropriate actions when deciding whether to wear a mask,” said Lisa Maragakis, an infectious-disease epidemiologist at Johns Hopkins University School of Medicine. “There is no way to know who is vaccinated and who is not in most scenarios. The likely result is that almost no one will wear a mask.”

The risk for people who have not yet been vaccinated, including millions of adolescents and children, “is going to dramatically increase as the rest of the population abruptly drops masking,” Maragakis added.

Some said the instructions might be more effective if paired with a way to verify people’s vaccination status, since that would give peace of mind to those gathering indoors and provide a real incentive to those yet to get the shots. Biden administration officials have said vaccine passports would be left to the private sector, and numerous Republican-controlled states have moved to ban such tools.

Besides children, those potentially at greater risk include essential workers who interact with the public, and millions of immunocompromised people who may have been vaccinated but whose bodies might not mount a full immune response, say doctors and labor groups.

“It came much faster than we expected,” said Emily Blumberg, director of transplant infectious diseases at Penn Medicine. “We are not ready for them to be as free as the CDC guidance allows.”

The CDC defended the guidance, which allows vaccinated people to go without masks or physical distancing in many cases, even when they are indoors or in large groups. Officials pointed to a study released Friday of nearly 2,000 health-care workers across 25 states that showed the two mRNA vaccines reduced the risk of covid-19 by 94 percent.

“This report provided the most compelling information to date that coronavirus vaccines were performing as expected in the real world,” CDC Director Rochelle P. Walensky said in a news release. “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.”

Two other studies were key: A March report in Nature Medicine showed substantially reduced viral loads among Israelis infected with the coronavirus after their first dose of the Pfizer-BioNTech vaccine. Lower viral load is thought to impede transmission of the virus, which would suggest vaccinated people do not spread the virus as easily. And a May 6 JAMA study found that a second Pfizer dose protected against even asymptomatic infections among fully vaccinated health-care workers.

“Preventing asymptomatic infections, I mean, ‘Wow,’” Henry Walke, a CDC medical officer who has been leading the agency’s coronavirus response since July, said in an interview. “That gave us a lot more confidence.”

Walke recommended to Walensky a change in the guidance, and she made the decision Monday evening, said a CDC official who spoke on the condition of anonymity to share internal discussions.

Federal officials stressed that state and local regulations, as well as rules maintained by businesses, still held sway, but representatives for front-line workers said the CDC’s backing was important for compliance.

“I think it ultimately means that we are going to become the vaccination police now,” said Marc Perrone, president of the United Food and Commercial Workers International Union, whose 1.3 million members include meatpackers and nursing home employees.

Susana Morales, a primary care physician at Weill Cornell Medicine, where she is vice chair for diversity, said the CDC’s new guidance did not square with its more tailored recommendations for some front-line jobs.

“When you think about why a lot of people of color and other folks were infected, it was because of being front-line workers,” she said Friday, minutes after learning that a friend’s father had died from the coronavirus. She urged federal, state and local officials “to think about exactly what to recommend for high-risk workplaces.”

Without a standardized system of vaccine certification, as has been adopted in Israel and is planned in Britain, officials said they would have to operate in the faith that unvaccinated people would wear masks, or rely on businesses to make their own rules.

Meanwhile, some of the lowest immunization rates in the country are in communities without mask mandates, calling into question the effectiveness of the change in federal guidance as an incentive for the unvaccinated.

It was mostly in Democratic-led states that mask mandates remained in effect when the CDC shifted its stance. A handful of states, including Virginia and Oregon, moved immediately to make their policies mirror the federal recommendations, while others, including Massachusetts and California, held off and said they would review the updated guidance.

The change was greeted with a shrug in places that already had lifted mask mandates — or never instituted them. “That horse has already left the barn,” said Susan Hassig, an infectious-disease specialist at Tulane University.

Many large businesses, including Target and Home Depot, said they would continue requiring employees and customers to wear masks. But others changed course. Walmart and Trader Joe’s both said vaccinated customers could shop without masks, unless state or local law says otherwise, without providing proof of vaccination. Walmart also will begin asking workers about their vaccination status and allowing the vaccinated to work without masks, while Trader Joe’s is maintaining its requirement for employees.

Lorraine M. Martin, president of the National Safety Council, a group of 16,000 U.S. businesses and other groups, said that since few companies are mandating employee vaccinations, they need to ensure workplace safety by requiring masks, including for those accustomed to going without masks elsewhere. Employers “have to play it safe so everyone continues to be safe,” she said.

The guidance is encouraging in paving the way for a fuller reopening, said Larry Lynch, a senior vice president at the National Restaurant Association. But restaurants that serve both vaccinated and unvaccinated patrons are in a tough spot. They will “still need to work with their state and local regulators to ensure they are in line with all other mandates,” he said.

Meanwhile, pediatricians and other doctors said parents of unvaccinated children and people with weakened immune systems should wear masks and remain vigilant, especially until it is clear how the recommendations affect viral transmission.

“Until younger children are eligible to be vaccinated for the covid-19 vaccine, they should continue to wear face masks when they are in public and around other people,” said Yvonne Maldonado, chair of American Academy of Pediatrics’s committee on infectious diseases.

On Twitter and other social media, teachers, parents of children under 12, people with immune-system problems and other vulnerable groups expressed frustration that the guidance might leave them with less — not more — freedom. That’s because the CDC’s announcement did not suggest any way to distinguish the vaccinated from the unvaccinated.

“I feel like everyone just forgot about all the unvaccinated little kids and their parents,” one parent tweeted.

Another quipped: “CDC Guideline: Don’t have a kid under 12.”

The new guidance went well beyond recommendations issued two weeks ago that eased mask-wearing in outdoor settings while stressing the continuing risk of transmission indoors. The update followed intensifying criticism from some lawmakers and public health experts that the CDC was being too conservative as access to the shots expanded, and risked sending mixed signals about the effectiveness of the vaccines.

“I think there’s no perfect time to issue guidance like this,” said Nancy J. Cox, a virologist and former CDC official. “So if you were being really cautious, you would wait until we got to about 20,000 cases per day. But we’re at a point where we need to be messaging about the benefits of the vaccine in a clearer, more convincing way. This is one step toward that.”

But the change prompted criticism that the agency might be motivated as much by political pressure as by science.

“The CDC, which is supposed to be our steady force based upon science, is lurching from extreme overcaution to abandoning all caution,” said Lawrence O. Gostin, a health law professor at Georgetown University Law Center. “And this all happened in a matter of weeks when the science really hasn’t changed.”

Gostin also cast doubt on the notion that the move would spur more people to get vaccinated.

“There’s zero behavioral evidence that a move like this would encourage people to be vaccinated,” he said. “It’s much more likely to encourage people to take their mask off.”

Others, such as Scott Harris, the top health officer in Alabama, disagreed, saying the agency’s emphatic endorsement of vaccines’ effectiveness might convince some doubters. Alabama has some of the lowest levels of immunization nationwide.

Clay Marsh, West Virginia’s coronavirus coordinator, said he understood the move as a vote of confidence in the vaccines, especially with the rarity of breakthrough, or post-vaccination, infections. Nonetheless, he was “surprised by the rapid movement,” he said.

States still requiring masks indoors raced to respond. Connecticut Gov. Ned Lamont (D) said he would adopt the CDC’s policy when he lifts other restrictions on May 19. As for how to tell whether someone has been cleared to go without a mask indoors, he said, “At this point, I think people are going to self-attest. I hope we can count on them to do the right thing.”

In New Jersey, however, Gov. Phil Murphy (D) said Friday the state would stick with its indoor mask mandate which had helped curbed the spread of the virus. “With all respect to the CDC, we have made extraordinary progress,” he said while touring vaccination site with Whoopi Goldberg.

There was variation within states, too. Pennsylvania was among the states that moved Thursday to follow the CDC’s guidance, but authorities in Philadelphia said they were still reviewing the matter.

“We understand everyone’s desire to stop using masks, but masks have been one of the most effective tools we have in stopping the spread of covid-19," said James Garrow, a city health department spokesman.

The differences are even more stark in jurisdictions that still have outdoor mask requirements. Among them is Brookline, Mass., where an advisory panel has been directing the town’s approach. One of its members, Natalia Linos, said she would like to see the town follow Massachusetts’ example by dropping the outdoor requirement.

But Linos, executive director of Harvard University’s FXB Center for Health and Human Rights, said the CDC’s new guidance, even if based in science, was not explained effectively.

“I’m a mom of three kids under 8, and if I were a single mom, I would need to bring them into the grocery store,” she said. “Even if 75 percent of people in the store were vaccinated, I would feel anxious.”

Fenit Nirappil, William Wan, Akilah Johnson and Amy Goldstein contributed to this report.