WASHINGTON — The Centers for Disease Control and Prevention said on Thursday that people who are fully vaccinated no longer have to wear masks in many indoor situations. The new guidance reflects evidence that vaccines are highly effective in preventing serious and critical cases of COVID-19, the disease caused by the coronavirus.

“Anyone who is fully vaccinated can participate in indoor and outdoor activities large or small without wearing a mask or physical distancing,” CDC Director Rochelle Walensky said. She looked relieved as she made the announcement, in a break from the mostly somber comportment of public health officials since the pandemic began.

A new notice on the agency’s website explains that masks should still be worn on “planes, buses, trains, and other forms of public transportation,” as well as in “correctional facilities and homeless shelters.”

Thursday’s guidance represented a sharp break with the incremental changes CDC had been making in recent weeks. In late April, for example, the agency revised its outdoor mask guidance, but it drew so many distinctions — between vaccinated and unvaccinated people, small gatherings and large ones — that some people were confused.

The debate about masks will almost surely continue, since CDC guidance is not law; states and localities can make their own rules, as can businesses like restaurants. But in her remarks on Thursday, Walensky clearly sought to extricate the agency she leads from the mask culture wars.

“If you are fully vaccinated, you can start doing the things you had stopped because of the pandemic,” she said. “We have all longed for this moment, when we can get back to some sense of normalcy.”

Walensky cited the “scientific data on the performance of our vaccines and our understanding of how the virus spreads” as reasons for lifting the mask guidance. Not only do the vaccines prevent people from falling ill with COVID-19, they also prevent people from spreading the virus to others who might then be sickened.

The surprisingly aggressive revision also comes as the agency faces criticism that it is not moving quickly enough in outlining the benefits of life after vaccination. For many people, no longer having to wear a mask will be foremost among those benefits.

Walensky noted that the vaccines now being administered are effective against various circulating variants of the coronavirus, including some that are more transmissible and can cause more serious illness.

“This is an exciting and powerful moment,” Walensky said. Her remarks were seemingly tailored not only to the CDC’s critics but also to people who have not yet received a coronavirus vaccine. After the final dose of the vaccine has been administered and the mandatory two-week period for antibodies to develop passes, she said, “you can shed your mask.”