The guidance was a seemingly abrupt reversal from guidance the agency issued in April when it eased mask-wearing guidelines for the outdoors. Under those recommendations, Americans who were fully vaccinated did not have to cover their faces unless they were in large crowds.
A month later, the CDC eased its guidance further for the fully vaccinated, saying that mask-wearing was not absolutely necessary in crowds and in most indoor settings.
The guidance still recommended that masks be worn in crowded indoor settings, like buses, planes, and hospitals but it cleared the way for reopening workplaces and other venues. Additional guidance said fully vaccinated people did not have to wear masks at schools.
These recommendations came as deaths and hospitalizations were on the decline. But then the delta variant hit. At the beginning of summer, a mutated more transmissible version of the coronavirus began spreading across the country, especially in areas with low vaccination rates.
Now, the CDC says the delta variant can spread even among those who have been vaccinated. It is recommending that, regardless of vaccination status, teachers, staff, students and visitors at schools wear masks while indoors.
The CDC’s latest guidance applies in parts of the country experiencing at least 50 new cases per 100,000 people in the week prior – guidance that applies to 60% of U.S. countries. As of Tuesday, the U.S. is averaging more than 57,000 cases a day and 24,000 COVID-19 hospitalizations.
The agency has not published the data on which it is relying, but CDC Director Dr. Rochelle Walensky said it was "concerning enough that we feel like we have to act."
Anticipating a pushback, Walensky acknowledged that many Americans are weary of the pandemic and do not want to return to prevention measures. But she said new scientific information forced the decision to change the guidance again.
"This is not something that I took lightly," she said.
Unsurprisingly, the guidance has been polarizing. Some public health experts are broadly in favor of masks but felt the CDC did a disservice by not issuing a call for Americans to document their vaccination status, in turn giving free rein for the unvaccinated to do what they want.
"It was completely foreseeable that when they (the CDC) made their announcement, masking would no longer be the norm, and that's exactly what's happened," Lawrence Gostin, a public health law professor at Georgetown University, said.
Meanwhile, Rep. Lauren Boebert, R-Colo., quipped that the CDC ought to change its name to the "Center of Demented Confusion."
South Dakota Gov. Kristi Noem, a Republican, said her office wouldn't be mandating any mask rules, and accused the CDC of being inconsistent.
"The CDC shifts their position AGAIN," Noem wrote on Twitter. "South Dakota's cases remain low. If you're worried about the virus, you're free to get vaccinated, wear a mask, or stay at home. But we won't be mandating anything. And the CDC's inconsistency doesn't help the American people."
The office of Florida Gov. Ron DeSantis – a longtime critic of the CDC’s mask policies – derided the latest guidance as ineffective.
"It isn’t based in science," DeSantis press secretary Christina Pusha told Fox News. "There is no indication that areas with mask mandates have performed any better than areas without mask mandates. In fact, this policy could actually backfire."
The Associated Press contributed to this report.