ATLANTA — The director of the Atlanta-based Centers for Disease Control and Prevention defended the agency’s reversal on mask-wearing recommendations in an interview with ABC News on Sunday.

Director Rochelle Walensky said the CDC weighed new data before announcing that Americans who are fully vaccinated can go without masks both outside and inside. Thursday’s announcement changed nearly all the previous masking and social distancing recommendations for Americans who are fully vaccinated.

“[W]e now have science that has really just evolved even in the last two weeks,” Walensky said on ABC’s “This Week.”

Walensky said new data shows vaccines are curbing the spread of the disease, and there is now widespread access to those vaccines.

The sudden change sparked praise from those eager to return to pre-pandemic life, particularly those who see the new guidelines as a way to reopen workplaces, schools and other venues that went dark during the pandemic.

Yet concerns have been raised from those who say there’s no easy way for businesses and others to determine who is fully vaccinated and who is not. Instead, many will have to rely on an honor system as many states and communities have already been lifting mask mandates amid improving virus numbers and as more Americans have been shedding face coverings after getting shots.

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The timing of the change has also faced questions. Just days earlier, Walensky had defended the agency’s strict mask guidance in front of a Senate committee where some Republicans on the panel described the CDC’s guidance as “unworkable.”

When pressed about the quick turnaround on the agency’s stance on mask wearing, Walensky said the agency was not giving in to pressure but instead needed time to review evolving science.

“I can tell you it certainly would have been easier if the science had evolved a week earlier and I didn’t have to go to Congress making those statements. But I’m delivering the science as the science is delivered to the medical journals,” she said.

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Walensky cautioned that even with the new guidelines, it was still too early to “declare victory,” but added that she was “cautiously optimistic” about the pandemic.

“We have to remain humble. We’ve had way too many curveballs in this pandemic come to us. But I am really cautiously optimistic that we are in a good place right now, that cases continue to come down,” she said.

But, she added, even though the guidance has changed, “there’s no need for everybody to start ripping off their masks.”

“There is no mandate to take it off. What we’re saying is, now this is safe,” she said. “Work at your own speed, work with your own family and your own businesses to remove them when necessary.”

To date more than 156 million Americans, or more than 47% of the population, have received at least one dose of COVID-19 vaccine, and more than 121 million are fully vaccinated.

Walnesky said everyone who is not fully vaccinated is still at risk. Walensky stressed that the guidance is up to individuals.

“If they’re vaccinated, they are safe. If they are not vaccinated, they are not safe. They should still be wearing a mask or better yet, get vaccinated,” she told “This Week” co-anchor Martha Raddatz. “We also need to say that this is not permission for widespread removal of masks,” she added on ABC.

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The CDC guidance still calls for masks in crowded indoor settings including buses, airplanes, hospitals, prisons and homeless shelters.

Several major chains, including CVS, Home Depot, Macy’s and supermarket giant Kroger Co., said they are still requiring masks in stores for the time being, though some said they are reviewing their policies.

But Walmart, the world’s largest retailer; Costco and Trader Joe’s said Friday that they won’t require vaccinated shoppers to wear a mask in U.S. stores, unless state or local laws say otherwise.

“I would imagine within a period of just a couple of weeks, you’re going to start to see significant clarification of some of the actually understandable and reasonable questions that people are asking,” Dr. Anthony Fauci, the face of the U.S. government’s pandemic response, said on Face the Nation.

Dr. Cecil Bennett, a Newnan-based family medicine physician, agreed with the CDC’s decision to reverse mask policies for fully-vaccinated Americans.

“I think the CDC had no choice but to make this move. Not because of the pressure from society, but because the data has been overwhelming,” said Dr. Bennett.

Bennett’s staff is fully vaccinated and he stopped requiring fully vaccinated patients to wear masks last week.

The Associated Press contributed to this report.