Fully vaccinated people no longer have to wear masks outdoors and in most indoor settings to protect against COVID-19, the Centers for Disease Control and Prevention announced Thursday in a move that surprised health experts and officials while delighting some mask-weary Americans and confusing many.

“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 announced at a White House media briefing on COVID-19. “If you are fully vaccinated, you can start doing the things that you had stopped doing because of the pandemic.”

Pennsylvania health officials immediately updated their guidance to align with the CDC’s, while New Jersey officials said they were reviewing the new advice.

“Today’s guidance from the CDC affects only people who are fully vaccinated,” said Pennsylvania acting Health Secretary Alison Beam. “This is another incentive to get the vaccine that is now easily and conveniently available. Once 70% of Pennsylvanians over 18 are fully vaccinated, we can completely lift the masking order.”

The Philadelphia Public Health Department, busy dealing with the resignation of its chief, Thomas Farley, over his decision years ago to cremate remains of a MOVE casualty, did not reply to a request for comment on the CDC announcement.

Walensky, at the CDC, said there are exceptions to the new advice. People with weak immune systems should speak with their doctors before putting their masks aside because vaccines might not be as effective for them. And the CDC will continue to recommend masks for vaccinated and unvaccinated people in certain crowded indoor settings, such as buses, planes, and hospitals.

But outside of very packed environments, Walensky said, the vaccines have proven to be highly effective at preventing both infection and serious illness from COVID-19, including against mutated versions of the virus that are more transmissible. She also cited a recent Israeli study that found, in the rare cases of infection after vaccination, the viral load is low, the infection is brief, and the infected person likely poses less risk of spreading the virus.

The new guidance comes as the aggressive U.S. vaccination campaign begins to pay off. New COVID-19 cases are at their lowest rate since September, deaths are at their lowest point since April 2020, and the test positivity rate is at the lowest point since the pandemic began.

» READ MORE: No need for a mask outdoors if you’re fully vaccinated, Fauci says

As of Thursday, 154 million Americans — more than 46% of the population — have received at least one dose of the COVID-19 vaccines, and more than 117 million are fully vaccinated. The rate of new vaccinations has slowed in recent weeks, but with the authorization Wednesday of the Pfizer vaccine for children aged 12 to 15, a new burst of doses is expected in the coming days.

Masking and physical distancing have been the cornerstones of pandemic precautions — and sources of resentment and resistance by those who feel their free choice has been usurped by government. Whether the huge shift in guidance will reduce vaccine hesitancy remains to be seen, but it may be an incentive.

“People shouldn’t need an incentive to get vaccinated,” said pediatrician Paul Offit, director of the Vaccine Education Center at Children’s Hospital of Philadelphia and a former vaccine adviser to the CDC. But if they do, “then this is their `get out of jail free’ card.”

Offit noted that some universities are already requiring students and faculty to be vaccinated if they want to return to campus. He hopes that schools will follow suit now that the Pfizer vaccine is authorized for children age 12 and up.

“Good people have lamented that this pandemic has imprisoned children,” he said. “Schools could say, ‘Welcome back to school without a mask if you are vaccinated.’”

Stephen Gluckman, a professor of infectious diseases and travel medicine at the University of Pennsylvania Perelman School of Medicine, speculated about the opposite scenario — that some people who haven’t yet gotten vaccinated will now decide they don’t need to.

While the vaccines are highly effective, none provide 100% protection. The goal of stopping the spread of the coronavirus can be achieved only if most of the population — experts estimate 75% to 90% — has immunity through natural infection or vaccination.

David Damsker, the director of the Bucks County Health Department, believes some people have been hesitant to get a shot because mask mandates made them doubt vaccine effectiveness. Now, he is optimistic more people will want to get vaccinated.

”This was a huge step,” said Damsker, adding that he wanted it to come sooner.

He thinks masking in public may be wise when people feel sick or are coughing — even for reasons unrelated to COVID-19 — but shedding this symbol of the pandemic is important.

”We have to get used to seeing people’s faces again,” he said.

Noting that more than half of Camden County residents are fully vaccinated, county commissioner director Louis Cappelli Jr. said the guidance paves the way toward a greater sense of normalcy. “Today will be marked as a turning point in our ongoing battle against COVID,” he said in a statement.

But even if local authorities lift precautions, many questions remain to be answered. How, for example, can retailers be sure that maskless patrons are actually vaccinated? What if some venues don’t want to continue masking and physical distancing requirements? What if fully vaccinated people avoid indoor activities where unvaccinated people may be in attendance?

The answers may depend more on psychological factors than science. Gluckman, at Penn, said vaccinated people who have remained diligent about mask-wearing and social distancing might not be ready to let go of that protective measure.

The suddenness of the CDC’s about-face may also raise concerns.

“It was such a reversal from what they said a few days ago,” said Lewis Nelson, chair of emergency medicine at the Rutgers New Jersey Medical School. “They were criticized heavily and appropriately for moving far too slowly.

“Sadly, the CDC has been politicized over the past few years. They’re still gaining their footing. Their decision-making used to be much more data-driven and more responsive to evidence.”

Gluck praised the sudden shift as “spectacular” but also said it was totally unexpected.

“I think it’s appropriate.” he said. “I don’t think it was imprudent to make this announcement, but there was not a whiff that this was going to happen until it did.”

Staff writers Sarah Gantz and Rob Tornoe contributed to this article.