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.

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