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 recommended 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, Dr. Rochelle Walensky, the agency’s director, said at a news briefing on Tuesday.
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.
Jen Psaki, the White House press secretary, said at a news conference on Tuesday that changing the guidance was crucial to “battling an ever-evolving virus” and that the Biden administration supported the effort.
“Their job is to look at evolving information, evolving data, an evolving historic pandemic, and provide guidance to the American public,” Ms. Psaki said.
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 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. 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, according to three federal officials with knowledge of the discussions.
Some public health experts welcomed the agency’s decision to revise its guidelines. 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.”
Whether state and local health 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, Mo., have already reinstated mask mandates in response to rising cases. But Arkansas, one of the states with the highest numbers, has retained a ban on mask mandates even as vaccination rates lag.
As recently as last week, a C.D.C. spokesman said that the agency had no plans to change its guidance, unless there were a significant change in the science. Researchers have begun to turn up disturbing new data.
The Delta variant is thought to be about 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 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, hinting that they also may spread it to others, according to three federal officials familiar with the matter.
Large so-called viral loads, particularly in the nose and throat, 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, 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 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.”
Understand the State of Vaccine Mandates in the U.S.
Dr. Walensky on Tuesday again urged people to get vaccinated, noting that the rise of cases and hospitalizations is greatest in places with low vaccination rates and among unvaccinated people.
She acknowledged that some vaccinated people can become infected with the Delta variant and may be contagious, but maintained that it was a rare event. But the C.D.C. is tracking only breakthrough infections that lead to hospitalization or death among vaccinated Americans. Officials have previously said that vaccinated people account for just 3 percent of hospitalizations.
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 spread.
“We’ve seen increasing numbers of breakthrough infections, and it seems like most of those may be happening in places where people are exposed to a lot of Covid,” said Dr. Scott Dryden-Peterson, an infectious disease physician and epidemiologist at Brigham & Women’s Hospital in Boston, who has been reviewing breakthrough infections in Massachusetts.
Vaccinated people — particularly people with weak immune systems or otherwise at high risk — should consider wearing masks even in areas of low transmission, he said: “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.”
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 four weeks ago. Hospitalizations have also been ticking up in nearly all states, and deaths have risen to an average of 275 per day.
“Given what we’re seeing, that’s absolutely needed right now to slow and curb transmission,” Dr. Robby Sikka, a physician who worked with the N.B.A.’s Minnesota Timberwolves, said of the new masking guidance.
“Not everyone who has a breakthrough infection will be at risk for transmission, but it’s imperative to note that there is a risk of transmission,” he said.
But Dr. Sikka noted that relying on states or localities to set masking rules will require more testing than is being done now to identify people with mild or asymptomatic infections. “That’s something that we’re probably not totally prepared to do,” he said.
Given that the virus seems likely to become endemic, permanently embedded in American life, federal officials need to articulate an even clearer plan for long-term masking, Dr. Nuzzo said.
“The question is, what are the off ramps for masking? It’s really important for us to define that,” she said. “If we want to continue to ask people to step up, we need to give them a vision of what we’re working toward.”