By Elizabeth Cohen and John Bonifield, CNN

(CNN) -- People fully vaccinated against Covid-19 do not need to wear masks or practice social distancing indoors or outdoors, except under certain circumstances, the director of the US Centers for Disease Control and Prevention announced Thursday.

"If you are fully vaccinated, you can start doing the things that you had stopped doing because of the pandemic," Dr. Rochelle Walensky said during a White House Covid-19 briefing. "We have all longed for this moment when we can get back to some sense of normalcy."

Calling it an "exciting and powerful moment," Walensky said the science supports the updated CDC guidance that "anyone who is fully vaccinated can participate in indoor and outdoor activities -- large or small -- without wearing a mask or physical distancing."

She cited three studies -- one from Israel and two from the United States -- that show vaccines work.

The Israeli study, which was published in the Journal of the American Medical Association, showed the vaccine was 97% effective against symptomatic Covid-19 and 86% effective against asymptomatic infection in over 5,000 health care workers.

There have been reports of "breakthrough" infections among vaccinated people in the United States -- a small number among more than 117 million people in the United States who are now fully vaccinated. Walensky noted that "the resulting infection is more likely to have a lower viral load, may be shorter in duration, and likely less risk of transmission to others."

Walensky's announcement has a few caveats. She warned that people who are immune compromised should speak with their doctors before giving up their masks.

The requirement to wear masks during travel -- on buses, trains, planes and public transportation -- still stands, Walensky said. Guidance for travel will be updated as science emerges.

She also said that "the past year has shown us that this virus can be unpredictable, so if things get worse, there is always a chance we may need to make a change to these recommendations."

People who develop Covid-19 symptoms, even those who are vaccinated, should put their mask back on and get tested, Walensky said.

The science is clear, too, for unvaccinated people, Walensky said: "You remain at risk of mild or severe illness, of death, or spreading the disease to others. You should still mask and you should get vaccinated right away."

But once someone is fully vaccinated -- two weeks after the final dose -- "you can shed your mask," she said.

Some people may choose to continue wearing masks even if they are fully vaccinated, and that's OK, federal Covid-19 response leaders said Thursday.

"People have to make their own personal choice," Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said during the White House Covid-19 briefing.

"There's absolutely nothing wrong with an individual who has a certain level of risk aversion, as we know the risk is extremely low of getting infected whether you're indoors or outdoors. But there are those people who don't want to take that bit of a risk and there's nothing wrong with that and they shouldn't be criticized."