Fully vaccinated people can have small gatherings indoors with other vaccinated people but need to wear masks in public, the C.D.C. says.

The Centers for Disease Control and Prevention on Monday issued long-awaited guidance to Americans fully vaccinated against Covid-19, freeing them to take some liberties that the unvaccinated should not, including gathering indoors with others who are fully vaccinated without precautions while still adhering to masking and distancing in public spaces.

The agency offered good news to grandparents who have refrained from seeing children and grandchildren for the past year, saying that fully vaccinated people may visit indoors with unvaccinated people from a single household so long as no one among the unvaccinated is at risk for severe disease if infected with the coronavirus.

That means fully vaccinated grandparents may visit unvaccinated healthy adult children and healthy grandchildren without masks or physical distancing. But the visit should be limited to one household: If the adult children’s unvaccinated neighbors drop by, the visit should move outdoors and everyone should wear masks and distance.

The recommendations arrived as state officials move to reopen businesses and schools amid a drop in virus cases and deaths. Federal health officials repeatedly have warned against loosening restrictions too quickly, including lifting mask mandates, fearing that the moves may set the stage for a fourth surge of infections and deaths.

The new advice is couched in caveats and leaves room for amendments as new data become available. The agency did not rule out the possibility that fully vaccinated individuals might develop asymptomatic infections and spread the virus inadvertently to others, and urged those who are vaccinated to continue practicing certain precautions.

Agency officials encouraged people to get vaccinated with the first vaccine available to them, to help bring the pandemic to a close and resume normal life. The agency emphasized that vaccines are highly effective at preventing “serious Covid-19 illness, hospitalization and death,” and said its guidance “represents a first step toward returning to everyday activities in or communities.”

“We know that people want to get vaccinated so they can get back to doing the things they enjoy with the people they love,” said Dr. Rochelle P. Walensky, director of the C.D.C. “There are some activities that fully vaccinated people can begin to resume now in the privacy of their own homes.”

Still, she added, “Everyone, including those who are vaccinated, should continue with all mitigation strategies when in public settings.”

Many more Americans will need to be fully vaccinated before mitigation measures can be suspended, she and other officials said, as the majority of Americans have yet to get the vaccine.

As of Sunday, about 58.9 million people have received at least one dose of a Covid-19 vaccine, including about 30.7 million people who have been fully vaccinated, according to the C.D.C. Providers are administering about 2.16 million doses per day on average.

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 a two-dose vaccine series of Pfizer-BioNTech or Moderna vaccine, and those for whom at least two weeks have passed since receiving a single dose of the Johnson and Johnson single-dose 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 so, then masking and other precautions are still be needed in certain settings to contain the virus, researchers have said.

There is also uncertainty about how well vaccines protect against emerging variants of the virus, and how long the vaccine protection lasts.

The C.D.C. said on Monday that “a growing body of evidence” suggests 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 could inadvertently transmit the virus.

Given the current state of research, the C.D.C. advised:

• Fully vaccinated Americans may gather indoors in private homes with one another in small groups without masks or distancing. Vaccinated people may gather in a private residence with unvaccinated people from a single household who are at low risk for developing severe disease if they contract the coronavirus, also without masks or distancing.

• Vaccinated Americans need not quarantine or get tested if they have a known exposure to the virus, as long as they do not develop symptoms of infection. If they do develop symptoms, they must isolate themselves, get tested and speak with their doctor.

• In public, vaccinated people must continue to wear masks and maintain social distance, and take other precautions, such as avoiding poorly ventilated spaces, covering coughs and sneezes, washing hands often and following any other protocols that are in place.

• Vaccinated people should continue to avoid large and medium gatherings, though the agency did not specify the gathering size with numbers.

The agency did not revise its travel recommendations, continuing to advise that all Americans refrain from travel unless absolutely necessary.

The advice is not legally binding, but the agency’s recommendations are usually followed by state public health officials. The recommendations seem likely to incentivize vaccination for many hesitant Americans by promising modest liberties after months of restrictions.