The Centers for Disease Control and Prevention has amended its guidance for fully vaccinated Americans, no longer recommending masks indoors or outdoors, including in crowds, CDC Director Rochelle Walensky announced at a White House briefing Thursday.
"If you are fully vaccinated, you can start doing the things that you had stopped doing because of the pandemic," Walensky said, announcing the sweeping change. "Anyone who is fully vaccinated can participate in indoor and outdoor activities, large or small, without wearing a mask or physical distancing."
The new recommendation, which carves out exceptions for buses, planes, hospitals, prisons and homeless shelters, will have significant implications for schools and businesses as the country begins to reopen.
"We have all longed for this moment when we can get back to some sense of normalcy," Walensky said.
The new recommendation is an about-face from guidance issued just 16 days earlier in which the CDC suggested masks should still be used indoors or in crowds even if people are fully immunized, which the CDC defines as two weeks after the final shot.
It also comes after Walensky faced criticism for the CDC being too slow to provide a path back to normalcy for fully vaccinated people, over 117 million of whom are in the U.S. Walensky has defended the CDC's approach as scientifically-based to ensure protection not just for individuals but also the entire U.S. population.
"In the last two weeks, the cases in this country have dropped by a third. In the last two weeks, we have had increasing available vaccine, and we now have available and eligible people between the ages of 12 and 15," Walensky said.
She also pointed to a "coalescence of more science that has emerged just in the last week" in three areas.
"One is the effectiveness of the vaccines in general in real world populations. One is the effectiveness against variants, which was just published last week. And then the effectiveness in preventing transmissibility," Walensky said.
While states will still have the choice to implement their own guidelines -- nearly half of all states still had some sort of state-wide mask mandate in place as of Thursday morning -- the new guidance will have immediate implications for offices, schools and public-facing businesses.
At the same time, mask enforcement for non-vaccinated people will be challenging and is likely to renew the debate over "vaccine passports," which some states have banned.
"With regard to what businesses, communities, schools we, of course, will be updating our guidance in many of these areas very shortly," Walensky said Thursday, but urged local public health departments to make the call in each community based on how many people are vaccinated and how many cases are in the area.
As for people who are fully vaccinated but feel uneasy walking into a public area where they won't know the vaccination status of others, Walensky said the science shows they're "protected."
"It is the people who are not fully vaccinated in those settings who might not be wearing a mask who are not protected, and it is those people that we are encouraging to get vaccinated and to wear a mask and to physically distance," Walensky said.
The CDC began to update its guidance for fully vaccinated adults last month, first giving the go-ahead for vaccinated people to hangout with other vaccinated people indoors without masks, and with unvaccinated people who were low-risk — allowing grandparents to see their grandchildren indoors without masks. Earlier this month, the CDC also announced that vaccinated people could ditch their masks outdoors so long as they’re not in crowds.
But a growing public health consensus that the vaccines are performing well against the variants commonly found in the U.S. and in curbing transmission suggested that the CDC could go further.
Meanwhile, lawmakers, particularly moderate Republicans, had also increased political pressure on the CDC and the Biden administration in recent days to further update their guidelines as an incentive for people to trust that life will get better as more people get vaccinated.
"It matters because it undermines public confidence in your recommendation. In the recommendations that do make sense. In the recommendations that Americans should be following," Maine Republican Sen. Susan Collins told Walensky at a congressional hearing on Tuesday.
Sen. Bill Cassidy, a Republican from Louisiana, told Walensky the American people have "just lost patience with us, with you guys."
He asked Walensky to put "real time into updating these things," acknowledging his frustration while also thanking her for her service.
Roughly 20% of Republicans say they will "definitely not" get the vaccine, a percentage that has decreased as more people get the vaccine but is still substantially higher than the share of Democrats and Independents, according to polling from the Kaiser Family Foundation.
At Thursday's briefing, Walensky said that while the CDC’s decision "may serve as an incentive for some people to get vaccinated, that is not the purpose."
"I want to be clear that we follow the science here," she said.
Walensky had begun hinting on Wednesday night that the CDC was nearing a decision on updated guidance, though she said they were still looking at data about transmission risks and variants.
"I have been completely forthcoming with respect to the science, with respect to our guidance, with respect to our numbers, with respect to our cases. And I really look forward to updating the guidance and providing the science that allows us to do so very soon," Walensky said on CNN.
And on Tuesday, when Biden similarly heard directly from governors about the impact updated guidance could have on the future of vaccinations, he also pledged guidance "soon."
"I would like to say that we have fully vaccinated people, we should start acting like it. And that's a big motivation to get the unvaccinated to want to get vaccinated," Utah Gov. Spencer Cox, a Republican, told Biden at a virtual meeting on Tuesday with a handful of other governors.
"We've gone a little slower to make sure we're exactly right in terms of the percent of the population that has been vaccinated, the adult population," Biden said, but noted that "we're gonna be moving on that in the next little bit."
Some experts, like Donald Milton, a professor at the University of Maryland’s School of Public Health, fully supported the CDC’s slow approach and intends on still wearing a mask until positive test rates fall substantially.
Milton, whose community in Maryland has a positive test rate of 4%, said he would continue to wear a mask indoors until it’s less than 1%. "I'm 95% protected against getting sick and ending up in the hospital, but it's not 100%," he said.
"I want to see us really suppress it, I don't want us to let up once again too early and see more people die," Milton said.
Other experts say the risk of transmission and infection is low enough for vaccinated people that they should begin to feel safe without masks, and that keeping the guidance as it is despite the increase of vaccines and drop in cases could erode trust.
"We have to be prepared to be flexible, and that means pulling back on controls when things look good," said Dr. Joe Allen, a professor at Harvard’s T.H. Chan School of Public Health.
At the same time, the country should "be prepared to implement more strict controls if things change," Allen noted.
Though it will undoubtedly be awkward to parse who is and isn’t vaccinated and assess safety with different groups.
Linsey Marr, a virus transmission expert at Virginia Tech, said it would be challenging to enforce because of the overlap between people that don’t want the vaccine and "those who resisted masking in the first place."
There will also be Americans who can’t get the vaccine for medical reasons, or who have less protection from the vaccines because they’re immunocompromised, who could now be at a greater risk of interacting with people who aren’t wearing masks.
"I think to date, the CDC has been trying to avoid that. And I understand that," said William Schaffner, a professor of preventive medicine and infectious diseases at Vanderbilt University Medical Center.
"But there are more and more people who've said, you know I've been vaccinated, these vaccines are supposed to be so darn effective, why can't I go back to a near normal life?" Schaffner said.
Schaffner said vaccinated people should feel confident indoors even in congregate settings, so long as they're not immunocompromised, and be comfortable ditching their masks around children.
"In order to start living again, we all have to start living with a small amount of risk," he said.
ABC News' Arielle Mitropoulos contributed to this report.