He doesn't believe lockdowns are likely to return, however.
WASHINGTON -- As the country grapples with a surge in the delta variant of the COVID-19 coronavirus, Dr. Anthony Fauci believes that lockdowns the country saw last year are likely to not return, though he warned "things will get worse" during an interview on ABC's "This Week."
"I don't think we're gonna see lockdowns. I think we have enough of the percentage of people in the country -- not enough to crush the outbreak -- but I believe enough to not allow us to get into the situation we were in last winter. But things are going to get worse," the nation's top infectious disease expert told "This Week" co-anchor Jonathan Karl on Sunday.
"If you look at the acceleration of the number of cases, the seven-day average has gone up substantially. You know what we really need to do, Jon, we say it over and over again and it's the truth -- we have 100 million people in this country who are eligible to be vaccinated who are not getting vaccinated. We are seeing an outbreak of the unvaccinated," he added.
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"From the standpoint of illness, hospitalization, suffering and death, the unvaccinated are much more vulnerable because the vaccinated are protected from severe illness, for the most part, but when you look at the country as a whole. And getting us back to normal, the unvaccinated, by not being vaccinated, are allowing the propagation and the spread of the outbreak which ultimately impacts everybody," Fauci said.
Concerns over the coronavirus resurged this week, as research about the outbreak of the virus in Provincetown, Massachusetts, indicated that the now-dominant delta variant may be able to spread among fully vaccinated people.
During an investigation of the outbreak, researchers learned that the amount of virus in the noses of vaccinated people experiencing a breakthrough infection was the same as in an unvaccinated person -- a concerning sign that vaccinated people can also spread the virus.
The data helped the CDC make its decision to bring mask guidelines back for vaccinated individuals in areas of high or substantial spread of the virus -- despite the fact that breakthrough cases in vaccinated individuals are overwhelmingly mild and do not result in hospitalization or death.
"That has much more to do with transmission," Fauci said of the new guidelines.
"You want them to wear a mask, so that if in fact they do get infected, they don't spread it to vulnerable people, perhaps in their own household, children or people with underlying conditions," Fauci said of the new guidance for the vaccinated.
President Joe Biden on Thursday also announced a new vaccine policy for all federal workers and onsite contractors, requiring them to "attest to their vaccination status," and will require anyone not fully vaccinated to wear a mask at work, regardless of where they are located, social distance and get tested once or twice a week.
Transportation Secretary Pete Buttigieg, who also spoke with Karl Sunday morning, pushed back on criticism from some unions representing those workers, who argue the new requirement is a violation of civil liberties.
"Well, this is about protecting lives. This is about setting a good example. And to be clear, employees have a choice," Buttigieg said in a separate interview on "This Week."
"Look, we have so many obligations in so many dimensions of employee safety, to make sure that this is a safe workplace. This is part of that. But it's also important, I think, for our federal workforce to lead by example because we're asking the whole country to do what it takes to make sure that we get beyond this pandemic. And this is a very important part of how to do it," he added.
But the new guidance and the president's acknowledgement Friday that "in all probability," the country could see new guidance and restrictions due to the surge has drawn the continued ire of some Republican governors, including Arizona's Doug Ducey, and Florida's Ron DeSantis, who argue that individuals should be able to make decisions about masking and vaccines for themselves.
"What is your answer to these ... Republican governors in some of the largest states in our country?" Karl asked Fauci.
"I respectfully disagree with them," Fauci said. "The fact is, there are things that are individual responsibilities that one has. And there are things that have to do with you individually, which also impact others and get the spread of infection that we're seeing now -- the surge in cases, Jon, is impacting everyone in the country."
"So in essence, you are encroaching on their individual rights because you're making them vulnerable. So you could argue that situation both ways," he added.
ABC News' Sony Salzman contributed to this report.