The Centers for Disease Control and Prevention recommended Tuesday that fully vaccinated Americans in parts of the country with “substantial or high transmission” of COVID-19 should return to wearing masks indoors, citing new data on the highly contagious Delta variant.

The CDC also said masks should be required of everyone inside K-12 schools while urging classrooms to return to full-time, in-person classes “with proper prevention strategies” in place. The agency recommended that all vaccinated people consider wearing a mask if someone in their household was unable to be protected by the vaccine, like those with compromised immune systems or children who are too young to receive a shot, regardless of transmission levels in their community.

The new guidance comes as a growing number of local and state health officials have already returned to mandating masks indoors, with cases of the virus surging among unvaccinated Americans. Some 63% of U.S. counties currently have a “high” or “substantial” spread of the virus, according to the CDC. 

Outside of the buses, planes, and other forms of public transportation governed by the Biden administration’s mask mandate, the CDC’s guidance isn’t binding on most local officials or private businesses and institutions. Some states had also banned local officials from levying their own mask mandates.

However, the agency’s reversal on wearing masks indoors will likely spur some companies and schools to revise their rules in parts of the country seeing outbreaks of COVID-19.

The CDC recommendations will result in masks being required in the White House again, starting Wednesday. Updated CDC data found substantial transmission rates in Washington, D.C.

In addition, in a memo obtained by CBS News, the Office of the Attending Physician says House members have to resume wearing masks in House buildings in D.C.

Some six in 10 American adults are fully vaccinated against COVID-19, according to the CDC. While the pace of Americans seeking out the shots remains at lows not seen since the earliest days of the vaccination rollout, the average of first doses has gradually increased over the past weeks, including in states with some of the highest rates of COVID-19 cases.

Walensky acknowledged the new masking recommendations would not be “a welcome piece of news” for vaccinated Americans, saying the decision was not “taken lightly.”

“I really do believe that masking right now, especially for those unvaccinated, is a temporary measure. What we really need to do to drive down these transmissions, in the areas of high transmission, is to get more and more people vaccinated, and in the meantime to use masks,” she said. 

Federal health officials have insisted for months that their guidance allowed for local “flexibility” in deciding to require masks in parts of the country facing outbreaks of the virus. However, since celebrating “independence from the virus” on July 4 at the White House, the Biden administration has faced growing calls to step up measures to curb the Delta variant as it surged to make up virtually all cases of the virus in some regions of the country.

In recent weeks, a growing body of scientific research has pointed to the potential risks the Delta variant could cause, including cases of so-called “breakthrough” infections among fully vaccinated people.

One federally-funded study from researchers at Houston Methodist Hospital, which has not yet been peer-reviewed, found the Delta variant appeared to cause “a significantly higher rate of vaccine breakthrough cases” than other circulating mutant strains of the virus. Another pre-print backed by the National Institutes of Health, from scientists at the Baylor College of Medicine, turned up an outbreak of the virus among some fully vaccinated attendees to an outdoor wedding.

Dr. Anthony Fauci, the president’s chief medical adviser, had disclosed over the weekend that walking back the mask guidance for fully vaccinated Americans was “under active consideration,” in an interview with CNN. 

The White House on Monday said current restrictions for international travelers would remain in place, citing concerns over the Delta variant and surges of cases among unvaccinated Americans. On Tuesday, President Biden urged Americans to follow the new recommendations and promised to lay out “the next steps” in the administration’s vaccination push on Thursday.

“I hope all Americans who live in the areas covered by the CDC guidance will follow it; I certainly will when I travel to these areas,” the president said in a statement.

Mr. Biden on Thursday is also expected to announce new guidance for federal employees. A memo distributed to federal workers on Tuesday said all federal employees, contractors and visitors in “areas of substantial or high community transmission” will be required to wear masks regardless of vaccination status. Unvaccinated employees will also be required to maintain social distance from others. Mr. Biden is also considering mandating regular testing for unvaccinated federal employees.

The Department of Veterans Affairs on Monday became the first federal agency to require employees to be vaccinated, with the VA secretary announcing all medical personnel will be required to be fully vaccinated in the next eight weeks.

Nancy Cordes, Weijia Jiang and Kristin Brown contributed reporting.