Federal health officials on Monday told millions of Americans now vaccinated against the coronavirus that they could again embrace a few long-denied freedoms, like gathering in small groups at home without masks or social distancing, offering a hopeful glimpse at the next phase of the pandemic.

The recommendations, from the Centers for Disease Control and Prevention, arrived almost exactly a year after the virus began strangling the country and Americans were warned against gatherings for fear of spreading the new pathogen.

Now the agency has good news for long-separated families and individuals struggling with pandemic isolation: Vaccinated grandparents may once again visit adult children and grandchildren under certain circumstances, even if they remain unvaccinated. Vaccinated adults may begin to plan mask-free dinners with vaccinated friends.

As cases and deaths decline nationwide, some state officials are rushing to reopen businesses and schools; governors in Texas and Mississippi have lifted statewide mask mandates. Federal health officials have repeatedly warned against loosening restrictions too quickly, fearing that the moves may set the stage for a fourth surge of infections and deaths.

The new recommendations are intended to nudge Americans onto a more cautious path with clear boundaries for safe behavior, while acknowledging that most of the country remains vulnerable and many scientific questions remain unanswered.

“As more Americans are vaccinated, a growing body of evidence now tells us that there are some activities that fully vaccinated people can resume at low risk to themselves,” Dr. Rochelle P. Walensky, the director of the C.D.C., said at a White House news conference on Monday.

On Thursday, President Biden will make his first prime-time television address, noting the first anniversary of the pandemic’s onset and highlighting “the role that Americans will play” in getting the country “back to normal,” Jen Psaki, the White House press secretary, told reporters on Monday.

As of Monday, 60 million Americans had received at least one dose of a Covid-19 vaccine, and about 31.3 million had been fully vaccinated, according to a database maintained by The New York Times. Providers are administering about 2.17 million doses per day on average.

Mr. Biden has promised that there will be enough doses for every American adult by the end of May. C.D.C. officials on Monday encouraged people to be inoculated with the first vaccine available to them, emphasizing that the vaccines are highly effective at preventing “serious Covid-19 illness, hospitalization and death.”

Despite the rapidly accelerating pace of vaccination, the pandemic will not recede overnight, said experts who praised the detail and scientific grounding of the C.D.C. recommendations.

“This is not turning a switch on and off,” said Dr. Carlos del Rio, vice president of the Infectious Diseases Society of America. “This is more like turning a faucet — you slowly start turning the faucet off.”

Even so, “it’s welcome news,” he added. “It’s the first time they are saying you can do something, as opposed to saying everything you can’t do. It’s huge.”

The new guidelines provide much-needed advice to individuals who are still reluctant to resume in-person, face-to-face interactions even after being vaccinated, said Vaile Wright, senior director for health care innovation at the American Psychological Association.

About half of all adults are anxious about re-entering normal life, including 44 percent of those who have been fully vaccinated, Dr. Wright said, citing soon-to-be published research from the American Psychological Association. “What drives that discomfort is the level of uncertainty,” she said.

“It’s really hard to know what’s safe and what’s not safe. When we can get some science-informed information out to people — ‘Here’s what you can do, but we still recommend doing this’ — that gives people what they need to make informed decisions about keeping them and their families safe.”

In the new guidance, federal health officials advised that fully vaccinated Americans may gather indoors in private homes in small groups with other fully vaccinated people, without masks or distancing.

They may gather with unvaccinated people in a private home without masks or distancing so long as the unvaccinated occupy a single household and all members are at low risk for developing severe disease should they contract the virus.

For example, vaccinated grandparents may visit unvaccinated healthy adult children and healthy grandchildren without masks or physical distancing.

Asked whether family members who are vaccinated should kiss and hug children and grandchildren who are not vaccinated, Dr. del Rio said yes but advised caution: “I wouldn’t overdo it.”

In public areas and in places like restaurants or gyms, vaccinated people should continue to wear masks, maintain social distance and take other precautions, such as avoiding poorly ventilated spaces, covering coughs and sneezes, and often washing their hands, C.D.C. officials said.

The C.D.C.’s advice is aimed at Americans who are fully vaccinated, meaning those for whom at least two weeks have passed since they received the second dose of the Pfizer-BioNTech or Moderna vaccines or a single dose of the Johnson & Johnson vaccine.

What is safe for newly vaccinated Americans and their unvaccinated neighbors and family members has been uncertain in large part because scientists do not yet understand whether and how often immunized people may still transmit the virus. If they can, then masking and other precautions are still needed in certain settings to contain the virus, researchers have said.

The C.D.C. said on Monday that research indicated that people who are fully vaccinated are less likely to have asymptomatic infections and “potentially less likely to transmit the virus that causes Covid-19 to other people.” Still, the agency did not rule out the possibility that they may inadvertently transmit the virus.

There is also uncertainty about how well vaccines protect against new variants of the virus that are more transmissible and possibly more virulent, as well as about how long the vaccine protection lasts. Some of the variants carry mutations that seem to blunt the body’s immune response.

The C.D.C. advised that vaccinated Americans do not need to quarantine or get tested if they are exposed to the virus, unless they develop symptoms of infection. If they do so, they should isolate themselves, get tested if possible and speak with their doctors.

Vaccinated Americans should not gather with unvaccinated people from more than one household, and should continue avoiding large and medium-size gatherings. (The agency did not specify what size constitutes a large or medium-size gathering.)

The guidance is slightly different for fully vaccinated residents of group homes and incarcerated individuals, who should continue to quarantine for 14 days and be tested if they are exposed to the virus, because of the higher risk of transmission in such settings.

Vaccinated workers in high-density settings like meatpacking plants do not need to quarantine after an exposure to the coronavirus, but testing is still recommended.

The C.D.C. did not revise its travel recommendations, continuing to advise that all Americans stay home unless necessary. Dr. Walensky noted that virus cases had surged every time there had been an increase in travel.

“We are really trying to restrain travel,” she said. “And we’re hopeful that our next set of guidance will have more science around what vaccinated people can do, perhaps travel being among them.”

The new guidelines clearly detail the rewards of vaccination and are likely to motivate even more Americans to seek immunizations and curb lingering vaccine hesitancy, said Dr. Rebecca Weintraub, an assistant professor of global health and social medicine at the Harvard Medical School.

“You can resume an activity that many people are yearning for — to be in proximity with those they love, in small gatherings where you can see each other smile and give each other a hug,” Dr. Weintraub said.

“It’s been well studied that anticipation is a significant component of joy,” she added. “These guidelines help each person coming in for a vaccine anticipate future joy. As a physician and vaccinator, I’m thrilled.”

Noah Weiland contributed reporting.