The head of the U.S. Centers for Disease Control and Prevention (CDC) on Sunday defended the decision to ease mask-wearing guidance for fully vaccinated people, stressing that increasing political pressure had nothing to do with the abrupt shift in guidelines.

"I'm delivering the science as the science is delivered to the medical journals. And it evolved," CDC Director Rochelle Walensky said on FOX News Sunday. "I deliver it as soon as I can when we have that information available."

Under the new guidelines released last week, fully vaccinated people – those who are two weeks past their final dose of a COVID-19 vaccine – can quit wearing masks outdoors in crowds and in most indoor settings and give up social distancing.

However, partially vaccinated or unvaccinated people should continue wearing masks, the agency said. The guidance also still calls for masks in crowded indoor settings including buses, airplanes, hospitals, prisons and homeless shelters.

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.

"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.

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.

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

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 curve balls 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."