The Centers for Disease Control and Prevention loosened Covid-19 guidelines on Thursday, freeing schools and businesses from the onus of requiring unvaccinated people exposed to the virus to quarantine at home.

The changes are a sharp move away from measures such as social distancing requirements and quarantining, which had polarized much of the country, and effectively acknowledge the way many Americans have been navigating the pandemic for some time. The agency’s action comes as children across the country return to school and many offices have reopened.

“We know that Covid-19 is here to stay,” Greta Massetti, a C.D.C. epidemiologist, said at a news briefing on Thursday. “High levels of population immunity due to vaccination and previous infection, and the many tools that we have available to protect people from severe illness and death, have put us in a different place.”

The C.D.C.’s new guidelines come after more than two years of a pandemic in which more than one million Americans have died. With the highly contagious BA.5 subvariant of Omicron spreading, the United States is recording more than 100,000 cases and nearly 500 deaths a day on average.

But many Americans dispensed with practices such as social distancing, quarantine and mask-wearing long ago.

“I think they are attempting to meet up with the reality that everyone in the public is pretty much done with this pandemic,” said Michael T. Osterholm, an infectious disease expert at the University of Minnesota, referring to the C.D.C.

The agency has been working for months on the new guidance, which builds on previous recommendations issued in February, when the agency shortened isolation times for many Americans. The C.D.C. said it is making changes now because vaccination and prior infections have granted many Americans some degree of protection against the virus, and treatments, vaccines and boosters are available to reduce the risk of severe illness.

The changes shift much of the responsibility for risk reduction from institutions to individuals. The C.D.C. no longer recommends that people stay six feet away from others. Instead, it notes that avoiding crowded areas and maintaining a distance from others are strategies that people may want to consider in order to reduce their risk.

And the recommended prevention strategies no longer draw a distinction between people who are up-to-date on their vaccinations and those who are not, streamlining a complicated set of rules that could be difficult for schools and businesses to navigate.

People who are exposed to the virus no longer must quarantine at home regardless of their vaccination status, although they should wear a mask for 10 days and get tested for the virus on Day 5, according to the new guidelines. Contact tracing and routine surveillance testing of people without symptoms are no longer recommended in most settings.

Instead of focusing on slowing transmission of the virus, the recommendations prioritize preventing severe illness. They emphasize the importance of vaccination and other prevention measures, including antiviral treatments and ventilation.

The guidelines around masking — which recommend that people wear them indoors in places where community Covid-19 levels are high — have not changed.

And people who test positive for the virus should still isolate at home for at least five days. Those who had moderate or severe illness, or are immunocompromised, should isolate through Day 10.

The agency also addressed the rebound infections that some people reported after taking the antiviral treatment Paxlovid; if symptoms return, people should restart the clock on isolation, the C.D.C. said.

Many health experts praised the new guidelines as representing a pragmatic approach to living with the virus in the longer term.

“I think this a welcome change,” said Amesh Adalja, a senior scholar at the Johns Hopkins Center for Health Security. “It actually shows how far we’ve come.”

The new guidelines will also be easier for the public to follow, he added.

But the pandemic has not ended, experts noted, and more stringent measures may be needed in the event of new variants or future surges.

While nearly all Americans are now eligible to be vaccinated, many are not up to date on their shots. Just 30 percent of 5- to 11-year-olds and 60 percent of 12- to 17-year-olds have received their primary vaccine series nationwide. Among adults 65 and older, who are at highest risk of severe illness, 65 percent have received a booster. Critical therapeutics, such as antiviral treatments, remain difficult for many to access.

“Obviously, we have to do more work to make sure that more people avail themselves of the protection that those tools have to offer and that more people can access those tools,” said Jennifer Nuzzo, director of the Pandemic Center at the Brown University School of Public Health. “I do think there’s been an overall dial-back in the ground game that’s needed to get people vaccinated.”

The guidance moves away from sweeping, population-level precautions to more targeted advice for vulnerable populations and specific high-risk settings and circumstances.

For instance, the guidelines note that schools may want to consider surveillance testing in certain scenarios, such as for when students are returning from school breaks or for those who are participating in contact sports.

Unvaccinated students who are exposed to the virus will no longer need to test frequently in order to remain in the classroom, an approach known as “test to stay.” The C.D.C. no longer recommends a practice known as cohorting, in which schools divide students into smaller groups and limit contact between them to reduce the risk of viral transmission.

Health experts said the change in guidance was particularly welcome as students head back to school, a setting in which quarantines had been especially disruptive.

“This really will help to minimize the impact of Covid-19 on education,” said Christina Ramirez, a biostatistician at the University of California, Los Angeles.

Mercedes Carnethon, an epidemiologist at the Northwestern University Feinberg School of Medicine, said that she did not view the changes, even the elimination of quarantines in favor of 10 days of masking, as a loosening of the agency’s guidance.

“We certainly know that wearing a high-quality mask is going to provide some of the strongest protection against spreading it to somebody else, and quarantine is logistically burdensome,” she said. “That could be seen as a relaxing of guidelines, but I think it’s a much more appropriate and targeted solution.”

Joseph Allen, a Harvard University researcher who studies indoor environmental quality, praised the new guidelines for putting more emphasis on improving ventilation.

“Good ventilation is something that helps reduce the risk of transmission that isn’t political and doesn’t require any behavior change,” he said.

Sheryl Gay Stolberg contributed reporting