(CNN) -- The US Centers for Disease Control and Prevention on Tuesday changed its masking recommendations as it grows more concerned over the Delta variant of COVID-19, urging vaccinated people in certain areas of the country to resume wearing masks indoors in public areas.
CDC Director Dr. Rochelle Walensky announced that the agency now recommends that people in areas with "high" or "substantial" COVID-19 transmission should resume wearing masks indoors. Nearly two-thirds of US counties have high or substantial transmission of COVID-19, according to CDC data; 46% of counties have high transmission and 17% have substantial transmission.
"In recent days I have seen new scientific data from recent outbreak investigations showing that that Delta variant behaves uniquely differently from past strains of the virus that cause COVID-19," Walensky told reporters.
"This new science is worrisome and unfortunately warrants an update to our recommendations," she said.
The new guidance comes about two months after President Joe Biden declared it a "great day in America" when the CDC revised its mask guidelines for vaccinated people and reflects a psychological setback for a country struggling to emerge from the grips of the pandemic. Wearing masks has been an issue fraught with politics, and the decision to revise mask guidelines will undoubtedly be met with harsh blowback from Republicans, especially on schools. Current CDC guidance states that all people age 2 and older who are not fully vaccinated should wear masks at school.
The guidance for people who are unvaccinated remains the same: Continue masking until they are fully vaccinated.
The CDC's latest guidance also recommends that community leaders encourage vaccination and mask-wearing to prevent further outbreaks in areas of substantial and high transmission. The agency recommends that local jurisdictions encourage universal indoor masking for all teachers, staff, students and visitors to schools, regardless of vaccination status.
Walensky said coronavirus vaccines reduced the risk of symptomatic infection seven-fold and reduced the risk of hospitalization 20-fold.
CNN earlier reported that top officials met on Sunday night to go over the new data and evidence regarding the transmissibility of the variant and breakthrough cases, according to a person familiar with the meeting.
Two months ago, when the CDC updated mask guidance saying most who are fully vaccinated could go without masks indoors, the guidance moved so quickly that administration officials were informed less than a day before. This time, the process is moving in a more methodical way as they decide how to proceed.
White House officials have repeatedly said it would be up to the CDC whether to change official guidance and that they would follow the lead of health and medical experts.
White House press secretary Jen Psaki said the health agency's decisions are based on a changing situation.
"The reality is we are dealing with a much different strain of this virus than we were even earlier in the spring, back in May, when the masking guidance was provided by the CDC at that time," Psaki told reporters. "That is their job. Their job is to look at evolving information, evolving data, an evolving historic pandemic and provide guidance to the American public. That's exactly what they will do and what they will provide specific details on later this afternoon."
A senior administration official said the decision was spurred by the more transmissible Delta variant.
"It's based on the fact that the Delta variant is clearly more transmissible than the prior ones," the official said.
New evidence found that the levels of virus found in breakthrough cases among the vaccinated people are similar to those found in unvaccinated individuals who contract coronavirus, raising concerns that vaccinated individuals may be able to spread the virus, the official said.
A separate health official said that, based on the latest data, officials believe vaccinated people only represent a small amount of coronavirus transmission and the overwhelming majority of spread is still by unvaccinated people. But they know breakthrough cases can happen and emerging data suggests some vaccinated people can spread it, which is the driving factor behind Tuesday's decision, as they continue to study it.
Versions of the virus that dominated in the past generally did not appear to be transmitted by people who had been fully vaccinated.
"When you get information about risks and how to mitigate risks, there's a public health obligation to let people know about it," the senior administration official said.
The White House is hoping that the new guidance will give local officials "a lot of cover" to implement new mask mandates where appropriate. Officials in several places, including Los Angeles, have been forced to return to earlier requirements on masks because of surging COVID-19 cases and hospitalizations among Americans who have refused to get vaccinated.
Dr. Anthony Fauci, the President's chief COVID-19 medical adviser and the director of the National Institute of Allergy and Infectious Diseases, said on Sunday the US was "going in the wrong direction" as the number of COVID-19 cases continues to rise, particularly among unvaccinated Americans.
Biden said last week that two dozen members of his COVID response team were examining the surge in cases and hospitalizations among the unvaccinated and determining whether new mask recommendations were necessary.
This story has been updated with additional background information.
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