Dr. Anthony Fauci acknowledged Tuesday that mask mandates by businesses are creating confusion in the wake of the Centers for Disease Control and Prevention’s announcement last week that fully vaccinated people no longer have to wear face coverings in most settings.

However, Fauci, the federal government’s top infectious disease expert and President Joe Biden’s chief medical adviser, said it is “perfectly reasonable and understandable” for businesses to continue to require masks.

“The problem and the issue is that we don't have any way of knowing who is vaccinated and who's not vaccinated,” Fauci said in an interview with ABC’s “Good Morning America.” “And I think that's where the confusion arises because there are some establishments who are saying: ‘Well, I'm going to have people coming into my establishment or store or what have you. Some are going to be vaccinated and some are not. I'm not going to know the difference. Some might be infected and might actually have a risk of infecting someone else.’”

As of Monday morning, 37.3% of Americans have been fully vaccinated, according to CDC data.

On Thursday, the CDC announced it was easing its guidance for fully vaccinated people, saying it was no longer necessary to wear masks indoors or outdoors in most settings or socially distance. Exceptions where masks should still be worn include on public transportation, at hospitals, at prisons and at homeless shelters.

Major retail chains such as Walmart, Target, Trader Joe’s and Starbucks have dropped their mask requirements for fully vaccinated customers except in areas where local and state laws still require face coverings. But other companies such as Home Depot, Apple, Whole Foods and Macy’s are still requiring masks for now.

All but eight states are in line with the CDC’s new guidance — and five of those holdouts have announced plans to end mask mandates for vaccinated people this month. 

When asked by ABC’s George Stephanopoulos if the CDC believes that “whichever choice the state makes is fine,” Fauci answered, “Not really.”

“If you look at the science that evolved over the last few weeks that prompted the CDC to make the recommendations that people who were vaccinated should feel safe and be able to go indoors and outdoors without wearing a mask relates to the evidence of how effective these vaccines are not only in protecting you against infection but, even if you have a breakthrough infection, the chances of you of transmitting it to someone else is extremely low,” Fauci said.