Originally Published: 27 JUL 21 07:24 ET

Updated: 27 JUL 21 11:31 ET

By Kaitlan Collins, John Harwood, Kevin Liptak, Jeremy Diamond and Kate Sullivan, CNN

(CNN) -- The US Centers for Disease Control and Prevention is expected to recommend everyone in K-12 schools wear a mask, regardless of their vaccination status, as it update its masking guidelines on Tuesday, according to an administration health official.

The CDC is also urging vaccinated people in certain areas of the country to resume wearing masks because of Covid-19.

People in areas with high or substantial Covid-19 transmission should resume wearing masks, the CDC is expected to say, according to sources familiar with the announcement. 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. CDC Director Dr. Rochelle Walensky is expected to announce the decision at a 3 p.m. ET briefing on Tuesday.

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.

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.

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 officials said.

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 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.