Revising a decision made just two months ago, the Centers for Disease Control and Prevention said on Tuesday that people vaccinated against the coronavirus should resume wearing masks in public indoor spaces in parts of the country where the virus is surging.

C.D.C. officials also called for universal masking for teachers, staff, students and visitors in schools, regardless of vaccination status and community transmission of the virus. With additional precautions, schools nonetheless should return to in-person learning in the fall.

The recommendations are another baleful twist in the course of America’s pandemic, a war-weary concession that the virus is outstripping vaccination efforts. The agency’s move follows rising case counts in states like Florida and Missouri, as well as growing reports of breakthrough infections of the more contagious Delta variant among people who are fully immunized.

“The Delta variant is showing every day its willingness to outsmart us,” Dr. Rochelle Walensky, director of the C.D.C., said at a news briefing on Tuesday.

The C.D.C. said Americans should resume wearing masks in areas where there are more than 50 new infections per 100,000 residents over the previous seven days, or more than 8 percent of tests are positive for infection over that period. Health officials should reassess these figures weekly and change local restrictions accordingly, the agency said.

By those criteria, all residents of Florida, Arkansas and Louisiana, for example, should wear masks indoors. Nearly two-thirds of U.S. counties qualify, many concentrated in the South.

The agency said that even vaccinated Americans in areas without surges might consider wearing a mask in public indoor settings if they or someone in their household has an impaired immune system or is at risk for severe disease, or if someone in the household is unvaccinated.

That includes vaccinated parents of children under age 12, who are currently ineligible for the shots.

C.D.C. officials were persuaded by new scientific evidence showing that even vaccinated people may become infected and may carry the virus in great amounts, perhaps even similar to those in unvaccinated people, Dr. Walensky acknowledged at the news briefing.

Data from several states and other countries show that the variant behaves differently from previous versions of the coronavirus, she added: “This new science is worrisome and unfortunately warrants an update to our recommendation.”

“This is not a decision we at C.D.C. have made lightly,” Dr. Walensky added. “This weighs heavily on me.” Americans are tired and frustrated, she said, and mental health challenges are on the rise.

After the agency’s announcement, White House staff were instructed to begin wearing masks again indoors. The Biden administration is considering requiring all federal employees to be vaccinated or to submit to regular testing and workplace restrictions, requirements similar to those being imposed in New York City and California.

“We have a pandemic because of the unvaccinated, and they’re sowing enormous confusion,” President Biden told reporters on Tuesday. “The more we learn about this virus and the Delta variant, the more we have to be worried and concerned. And there’s only one thing we know for sure — if those other hundred million people got vaccinated, we’d be in a very different world.”

The C.D.C. needed to revisit its recommendations, said Dr. Anthony S. Fauci, the administration’s lead adviser on the pandemic. “I don’t think you can say that this is just flip-flopping back and forth. They’re dealing with new information that the science is providing.”

The vaccines remain remarkably effective against the worst outcomes of infection with any form of the coronavirus, including hospitalization and death. But the new guidelines explicitly apply to both the unvaccinated and vaccinated, a sharp departure from the agency’s position since May that vaccinated people do not need to wear masks in most indoor spaces.

Those recommendations, which had seemed to signal a winding down of the pandemic, were based on earlier data suggesting that vaccinated people rarely become infected and almost never transmit the virus, making masking unnecessary.

But that was before the arrival of the Delta variant, which now accounts for the bulk of infections in the United States. And it may be followed by others. “The big concern is that the next variant that might emerge — just potentially a few mutations away — could evade our vaccine,” Dr. Walensky said.

Whether masks become ubiquitous again may depend on local surveillance and outreach efforts, which vary from state to state. Many Americans simply do not know what infection rates and positive test rates are in their area on a week-by-week basis.

Based on what scientists are learning about the Delta variant’s ability to cause breakthrough infections, “this is a move in the right direction,” said Dr. Celine Gounder, an infectious disease specialist at Bellevue Hospital Center in New York.

The American Federation of Teachers and the National Education Association, the two leading teachers’ unions, strongly endorsed the C.D.C.’s move to universal masking in schools.

“Masking inside schools, regardless of vaccine status, is required as an important way to deal with the changing realities of virus transmission,” said Randi Weingarten, president of the A.F.T. “It is a necessary precaution until children under 12 can receive a Covid vaccine and more Americans over 12 get vaccinated.”

Other union officials said the guidance did not go far enough, and would fail to protect frontline and essential workers in supermarkets, retail stores and meatpacking plants.

“A national mask mandate is the only way we can finally take control of this virus,” said Marc Perrone, president of the United Food and Commercial Workers International.

Whether state and local government officials are willing to follow the agency’s guidance is far from certain. And there is sure to be resistance from pandemic-fatigued Americans, particularly in regions of the country where vaccination rates are low and concerns about the virus are muted.

Some jurisdictions, like Los Angeles County and St. Louis County, have already reinstated mask mandates in response to rising cases. But officials in some communities in Los Angeles County have said they will not enforce a mandate. And the Missouri attorney general has filed a lawsuit against the city of St. Louis to stop the measure.

Businesses, too, are likely to find that new mask recommendations complicate plans to return to their offices in places where the virus is spreading and may necessitate new mandates for employees to get vaccines.

The Washington Post, for example, on Tuesday said it would require proof of vaccination as a condition of employment when workers return to the office in September, after hearing concerns from many employees about the emergence of coronavirus variants.

If businesses believe that such mandates would be beneficial, “we encourage them to do so,” Dr. Walensky said at the news briefing. “We’re encouraging, really, any activities that would motivate further vaccination.”

As recently as last week, a C.D.C. spokesman said that the agency had no plans to change its masking guidance, unless there was a significant change in the science. Now researchers have begun to turn up disturbing data.

The Delta variant is thought to be more than twice as contagious as the original version of the virus. Some research now suggests that people infected with the variant carry about a thousandfold more virus than those infected with other variants, and they may stay infected for longer.

C.D.C. officials were swayed by new research showing that even vaccinated people may carry great amounts of the variant virus in the nose and throat, suggesting that they also may spread it to others.

Large so-called viral loads may help explain reports of breakthrough infections in groups of vaccinated people. For example, an outbreak that began in Provincetown, Mass., after Fourth of July festivities there, has grown to include at least 765 cases, according to Steve Katsurinis, chair of the Provincetown Board of Health.

Of the 469 cases reported among Massachusetts residents alone, 74 percent were in people who were fully immunized, Mr. Katsurinis said.

Smaller clusters of breakthrough infections have been reported after weddings, family reunions and dinner parties. Some of the infected people had symptoms, but the vast majority were not seriously ill, suggesting that immunity produced by the vaccines quickly curbs the virus.

Vaccines “are not a force field,” said Jennifer Nuzzo, an epidemiologist at Johns Hopkins University’s Bloomberg School of Public Health. Instead, vaccination trains the immune system to recognize cells that become infected with the virus.

“The term ‘breakthrough infection’ is probably a bit misleading,” she said. “It’s probably more realistic that we talk about ‘breakthrough disease’ and how much of that is occurring.”

Dr. Walensky acknowledged that some vaccinated people can become infected with the Delta variant and may be contagious, but maintained that it was a rare event. So far vaccinated people account for just 3 percent of hospitalizations, officials have found.

Dr. Gounder and other experts said that it is unclear how often vaccinated people transmit the virus to others, but it may be more common than scientists had predicted as the original virus was spreading last year.

Vaccinated people — particularly those with weak immune systems or otherwise at high risk — should consider wearing masks even in areas of low transmission, said Dr. Scott Dryden-Peterson, an infectious disease physician and epidemiologist at Brigham and Women’s Hospital in Boston. “Masks can effectively reduce the amount of virus that we breathe in and prevent us from getting sick, and so they augment the impact of our vaccine. Almost everywhere in the U.S., it’s a good idea,” he said.

Infections have been rising swiftly in the United States, to more than 56,000 daily cases on average, as of Tuesday, more than four times the number a month ago. Hospitalizations have also been ticking up in nearly all states, and deaths have risen to an average of 275 per day.

Federal officials need to articulate clear plans for testing and long-term masking, experts said.

“The question is, what are the offramps for masking?” asked Dr. Nuzzo. “If we want to continue to ask people to step up, we need to give them a vision of what we’re working toward.”

The C.D.C. should have simply have told all Americans to wear masks indoors, said Ali Mokdad, an epidemiologist at University of Washington and former C.D.C. scientist.

“If you look at the country, every state is seeing a rise in transmission,” Dr. Mokdad said. “So why not say, ‘Everybody in the U.S. should be wearing a mask indoors?’ The whole country is on fire.”

Reporting was contributed by Roni Caryn Rabin, Neil MacFarquhar and Daniel E. Slotnick in New York, and Annie Karni and Sheryl Gay Stolberg in Washington.