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Dr. Anthony Fauci announced on Sunday that the Centers for Disease Control and Prevention is assessing new COVID-19 guidelines that would require face masks for those who are already vaccinated. Fauci, who is the chief medical advisor to President Joe Biden, said the change in mask guidance may be necessary because the United States is "going in the wrong direction" as far as COVID-19 cases.

During an interview on CNN's "State of the Union, Fauci cautioned, "And I have said, it is really a pandemic among the unvaccinated, so this is an issue predominantly among the unvaccinated, which is the reason why we're out there, practically pleading with the unvaccinated people to go out and get vaccinated."

Fauci expressed concern about the outbreak of the virulent delta variant, "And since we have 50 percent of the country [that] is not fully vaccinated, that's a problem, particularly when you have a variant like Delta, which has this extraordinary characteristic of being able to spread very efficiently and very easily from person to person."

Because of the delta variant, some local governments, such as Los Angeles County, have reimposed its mask requirement for people in public locations, including for those who are fully vaccinated. In May, the CDC issued a new guidance that said fully vaccinated Americans are no longer advised to wear face masks indoors or outdoors under most circumstances. However, Fauci said the CDC may walk back its mask guidance for those who received the COVID-19 vaccine.

"That's a local decision that's not incompatible with the CDC's overall recommendations that give a lot of discretion to the locals," Fauci told host Jake Tapper. "And we're seeing that in L.A. We're seeing it in Chicago. We're seeing that in New Orleans, because the officials there, many of them are saying, 'Even if you're vaccinated, it's prudent to wear a mask indoors.'"

Fauci revealed that mask mandates for vaccinated Americans are "under active consideration" by the CDC, and said that he is "part of the discussion."

Fauci praised Republicans, including Govs. Asa Hutchinson of Arkansas and Ron DeSantis of Florida, for fervently urging their constituents to get the coronavirus vaccine.

"What I would really like to see is more and more of the leaders in those areas that are not vaccinating to get out and speak out and encourage people to get vaccinated," Fauci said.

The director of the U.S. National Institute of Allergy and Infectious Diseases said a booster shot "might likely" happen, which would be for people with suppressed immune systems, such as organ transplant and cancer patients.

Over 163 million people, or 49.1% of the eligible U.S. population, are vaccinated, according to CDC data.