The nation's top infectious disease expert, Dr. Anthony Fauci, has defended the Centers for Disease Control and Prevention's (CDC) new mask mandate for people who already completed their cycle of immunization against COVID-19.

The new CDC mask mandate caused confusion among state and local health officials. CDC had updated its guidelines in mask-wearing, saying it is safe for fully vaccinated people to waive social distancing and go to most places without a mask may it be indoor or outdoor, Fox News reported.

Dr. Anthony Fauci said the new CDC mask mandate was based on the "evolution of science." He added that it could serve as an incentive for all people to get vaccinated.

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Confusion On New CDC Mask Mandate

CDC Director Dr. Rochelle Walensky had also defended the agency's decision on the new mask mandate. Walensky said the move was science-driven and an individual assessment of one's risk.

However, some states have insisted that they would still impose a mask-wearing requirement in their area of jurisdiction, like in Hawaii and Massachusetts.

The confusion stems from the information that there is no definitive way to track who has already received a vaccine that has forced businesses to work in believing an individual's statement, CNBC News reported.

The CDC has updated its mask guidance on Friday. It noted that masks are still required on planes and other public transportation.

Local leaders in New York, New Jersey, California, and Washington, D.C. are still seeking to review the new guidance before implementing the changes in their respective states.

On the other hand, Minnesota and North Carolina ended their mask requirements, ABC News Go reported. Michigan, Maryland, and Virginia had also followed suit in lifting their mask mandate for those already vaccinated.

Maryland Gov. Larry Hogan said that this was the day that many have been waiting and working with. Maine had decided to follow the new CDC mask mandate on May 24, while California Gov. Gavin Newsom said that the state would drop the mask mandates once the state fully reopens. 

Puerto Rico still requires people to wear face masks in public settings.

Critics are quick to question the decision as variants still threaten the spread of the contagious disease. They added that many regions in the U.S. are still struggling with vaccine hesitancy.

Herd Immunity

New COVID variants are spreading quickly, and the vaccination program is slow for herd immunity to take place anytime soon, according to The New York Times report.

Rustom Antia, an evolutionary biologist at Emory University in Atlanta, said that the virus is unlikely to go away. 

Dr. Anthony Fauci noted that people should forget herd immunity for a second, adding that the focus should be vaccinating more people for infections to go down.

The previous target immunity threshold was around 60 to 70 percent of the population. Now, experts are saying that herd immunity should be at least 80 percent.

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WATCH: Fauci Weighs In On CDC's New Mask Guidance For Fully Vaccinated - From NBC News