The new recommendations signal that the Biden administration and its medical advisers have decided that the lower fatality rate from covid-19 in a heavily vaccinated population permits a less demanding set of guidelines.

“The current conditions of this pandemic are very different from those of the last two years,” CDC epidemiologist Greta Massetti said Thursday in a briefing for reporters.

The virus has killed more than 1 million people in the United States since it arrived in early 2020. About 42,000 people with covid are hospitalized and the daily death toll is close to 500, according to a Washington Post seven-day average of daily trends. Those numbers, though quite a bit higher than in early spring, do not approach the dire figures of last winter, and CDC officials have repeatedly pointed to greater protection against the virus because of high levels of vaccine- and infection-induced immunity, coupled with the rollout of effective treatments that have reduced severe illness.

A report released Thursday by the agency explaining the guidance revisions said the more favorable circumstances allow public health officials to focus on “sustainable measures to further reduce medically significant illness as well as to minimize strain on the health care system, while reducing barriers to social, educational, and economic activity.”

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But the revision in guidance carries some risk, according to infectious-disease experts: Another fall and winter wave of cases, or the emergence of a new coronavirus variant, could call into question the wisdom of the CDC’s strategic pivot or hamper the agency’s ability to reimpose tougher guidelines.

As part of the changes, the agency is dropping its recommendation that people be screened or tested for covid in most settings. That change is likely to affect policies in workplaces, schools and day-care centers.

“When considering whether and where to implement screening testing of asymptomatic people with no known exposure, public health officials might consider prioritizing high-risk congregate settings, such as long-term care facilities, homeless shelters, and correctional facilities, and workplace settings that include congregate housing with limited access to medical care,” the CDC wrote in the report explaining the changes.

One CDC webpage, titled “How to Protect Yourself and Others,” has been extensively revised. It no longer states, for example, “If possible, maintain 6 feet between the person who is sick and other household members.” The new language is more nuanced, does not employ the 6-foot rule, and acknowledges that it may be impractical to stay away from a sick person: “In those situations, use as many prevention strategies as you can, such as practicing hand hygiene, consistently and correctly wearing a high-quality mask, improving ventilation, and keeping your distance, when possible, from the person who is sick or who tested positive.”

Under the new guidance, quarantine procedures have been relaxed: Unvaccinated people who have had close contact with someone who is infected no longer are advised to go through a five-day period of quarantine if they have not tested positive for the virus or shown symptoms.

Previous CDC guidance said people who had been exposed but were up to date on their coronavirus shots could skip the quarantine period. The new guidance expands that standard to everyone.

The updated CDC guidance does not call for dropping all precautionary measures. For example, people who have been exposed — but not confirmed to be infected — should still wear a mask and get tested at least five days after exposure.

People who test positive should continue to isolate immediately and stay home for five full days if positive. (“Isolation,” as opposed to “quarantine,” covers those known to be infected or who are symptomatic.)

The CDC did not call for a negative test before exiting isolation. Some infectious-disease experts have argued that a negative test offers direct evidence of a person’s potential to spread the virus, unlike the one-size-fits-all timeline. And the five-days standard has been criticized as too short. A recent study found that people continue to test positive on antigen tests for eight days, on average, after becoming infected.

“I think the question is, is the CDC finally saying, ‘Look, we’ve done what we can do to contain the most acute phases of this pandemic?’” said Jeanne Marrazzo, an infectious-diseases expert and clinician at the University of Alabama at Birmingham. “So are they just finally saying that it is time for us to sort of take a step back and think about putting this back to the individual person?”

The more relaxed guidelines are “a concession to realism, to the way that a lot of people are handling this,” said William Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health. He called the new guidelines “entirely reasonable,” but added, “My major concern is whether they will continue to be entirely reasonable given the unpredictable dynamics of the virus.”

The revision by the CDC of its coronavirus guidance is the most significant move by the agency since the massive outbreak of infections from omicron last winter. Omicron sickened tens of millions of people in a matter of weeks. The CDC had been recommending a 10-day isolation period up to that point, but omicron quickly decimated the labor force and the agency abruptly cut isolation guidance in half.

The CDC is not a regulatory body. Its guidelines do not have the force of law. But many government jurisdictions, businesses, schools and millions of people have tried to adhere to the agency’s guidelines during the pandemic.

Social factors and not just virologic ones have shaped the CDC’s approach. The agency’s director, Rochelle Walensky, has said the agency wants to offer practical recommendations that can, and will, be followed by a broad swath of the public. That means taking equity issues into account, because people do not have equal access to tests, or the same ability to work remotely or isolate from family members. The agency has said it wants to avoid unnecessary extension of isolation for people who test positive but are unlikely to still be infectious.

Public health experts, interviewed in recent days about the anticipated loosening of restrictions, said the changes reveal the Biden administration’s strategic shift toward a “live with the virus” approach.

Although the latest omicron subvariant, BA.5, has fueled a wave of infections in many parts of the country over the past two months, the great majority of those cases have not required hospitalization.

“The pandemic is in a very different spot” than it was last year, said David M. Aronoff, an infectious-disease expert and physician at Indiana University. “We know the majority of Americans have some immunity to SARS-CoV-2, either because they’re immunized by a vaccine or immunized by an infection.”

Thursday’s CDC action arrives as the agency’s latest data indicates the summer BA.5 wave has peaked. The share of the country living in the highest level of stress on the medical system has dropped from 61 percent to 45 percent during the past two weeks. The South, where 66 percent of the people live in high-covid areas, continues to be by far the most-affected area, but all four regions of the country showed declining covid intensity the past two weeks.

Many workplaces may use the latest guidance to stop routine testing, and may cut back on other preventive policies, Aronoff said. But if there is a spike in severe illness and deaths in the fall, “there has to be a willingness to ramp up or dial up our preventive strategies if needed,” Aronoff said.

Caitlin Rivers, an infectious-disease epidemiologist at the Johns Hopkins Center for Health Security, said it made sense to standardize the quarantine guidance regardless of vaccination status.

“The vaccines remain protective against severe illness, but the combination of variants and waning [immunity] have eroded their protection against infection,” Rivers wrote in an email. But, she added, “I feel strongly that people who are exposed should wear a mask and test between Days 5 and 7 to avoid infecting others.”

Rivers said she was sorry to see the agency drop its recommendation that testing be used to screen for the virus, calling it a “useful tool for identifying outbreaks early and monitoring the burden of disease in a population.” The expense can be a barrier, she said, but “absenteeism and losses in learning or productivity are also costly.”

Julia Raifman, an assistant professor at the Boston University School of Public Health, said what is most notable about the guidelines is what is missing.

“The CDC sets the bar on what should happen, like a speed limit,” Raifman wrote in an email. “Instead, we have the CDC establishing that there are no speed limits and making it very difficult for state and local governments to set better policies.”

There is no policy guidance “to reduce a bad surge and its harms to health as well as widespread disruption of education and work by turning on a mask mandate and increased testing,” Raifman said. CDC should “establish longer-term policies for living with less COVID and have policy preparedness to make surges less harmful and disruptive to health and the economy.”

The new guidance also provides additional information about the reliability of rapid antigen tests and the importance of testing more than once — what is known as serial testing. It comes amid evidence that many people who are symptomatic with an illness and suspect they have covid are still testing negative, initially, on the at-home tests.

The new guidance says people who are exiting isolation and don’t want to wear a mask for another five days, as the CDC recommends, can take two tests to reach mask-free status. One test should come no sooner than Day 6 (after symptoms appeared, or the person had that initial positive test result). The second test should come 48 hours later, the new guidance said. Two antigen tests with more than 48 hours between them provides “more reliable information because of improved test sensitivity,” the guidance said. People should have two consecutive negative tests to discontinue masking, the guidance said.

A University of Massachusetts research study, posted online but not yet published in a peer-reviewed journal, has spurred the government to embrace serial testing. The study showed that two rapid antigen tests conducted 48 hours apart were sensitive to 93 percent of the infections detected independently by the much more accurate PCR tests during the initial week of infection. But a single rapid test caught only 60 percent of infections among symptomatic people on the day they came up positive on the more sensitive PCR test, according to the study’s lead author, Apurv Soni, director of the program in digital medicine at the University of Massachusetts Chan Medical School.

“Public health implications of our findings suggest that people who are suspected to be infected with SARS-CoV-2 should exercise caution despite an initial negative rapid antigen-test and favor mask-wearing and avoiding crowded places,” the authors wrote. Given reports of people continuing to test positive after five days of isolation, the authors said their findings “support isolation for a longer period of time to prevent the potential of spread of SARS-CoV-2 to others.”

Even so, the CDC is not making major changes in its recommendation for people who are recovering from covid and want to know when they can exit isolation. That guidance says patients can end isolation five days after their first day of symptoms, so long as their symptoms have improved and they have been fever-free for at least 24 hours without fever-reducing medication. The CDC encourages people who become very sick or have weakened immune systems to isolate for 10 days.

In the coming days, the agency plans to consolidate hundreds of websites related to its covid response, each with different messages on testing, ventilation and masking in different settings.

Jacqueline Dupree and Dan Keating contributed to this report.