The Centers for Disease Control and Prevention on Tuesday announced new mask guidelines, urging Americans to wear masks in indoor public areas regardless of their vaccination status.

This recommendation comes at a time of growing concern about the highly contagious and deadly Delta variant, which has had a severe impact on unvaccinated Americans.

CDC Director Dr. Rochelle Walensky announced the updated mask guidelines, recommending wearing masks indoors for those who live in areas with "high" or "substantial" COVID-19 transmission.

According to CDC data, nearly two-thirds of U.S. counties have high or substantial transmission of COVID-19. Of the almost 3,000 counties, 46% have high transmission and 17% have substantial transmission.

Though vaccinated people represent a very small percentage of transmission, there is still concern they can carry the virus and transmit it to unvaccinated people who are more vulnerable to COVID and its variants, administration officials say.

Top officials met late Sunday to assess the new data and evidence regarding the transmissibility of the variant and breakthrough cases, a person familiar with the meeting told CNN.

White House press secretary Jen Psaki said that President Joe Biden is prepared to start wearing a mask again as health officials learn about the different strains of the virus.

"We are still in the midst of a once in a generation pandemic, battling an ever-evolving virus," Psaki said.

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