Top U.S. health officials are "looking very closely" at reports that a much higher percentage of younger Americans than expected need hospitalization as a result of contracting the coronavirus, Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, said Sunday.

Fauci was responding to new data from the Centers for Disease Control and Prevention, which, after studying more than 4,000 cases in the U.S., showed that about 40 percent of those who were hospitalized for the virus as of March 16 were ages 20 to 54. Among the most critical cases, 12 percent of intensive care admissions were among those ages 20 to 44, while 36 percent were for those 45 to 64.

About 80 percent of people in the U.S. who have died from COVID-19 were 65 and over, with the highest percentage among those over 85.

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Asked about the new numbers on CBS' "Face the Nation," Fauci said they highlighted "a very important critical issue that we're looking very closely at."

"It looks like there is a big difference between that demography from China and what we're seeing in Europe," Fauci said. Data based on China's outbreak suggested that older people were more at risk.

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"Now we have to look at the young people who are getting seriously ill from the European cohort and make sure that it isn't just driven by the fact that they have underlying conditions, because we know that underlying conditions — all bets are off no matter how young you are if you have an underlying, serious medical condition. You're going to potentially get into trouble," Fauci added.

"But if they don't have underlying conditions, that will be something we will have to really examine as to why we're seeing it here but we didn't see it in China. So we're going to look at that very closely."

Speaking on CNN's "State of the Union," Rep. Alexandria Ocasio-Cortez, D-N.Y., said young people need to take the risks seriously.

"I've been speaking about this for over a week now, and if you are a young person in America today, you need to stay home," she said. "There was so much messaging about how coronavirus is only impacting older people and that younger people don't have to worry about it for their personal health. Well, let me tell you something, in the state of New York, about 55 percent of our cases are with folks 18 to 49."

"And when you have that ... you are able to be directly impacted," she added. "You're going to get your mom sick. You're going to get your grandparents sick. You're going to get people you care for sick if you are asymptomatic. So you may not think that you have it and you very well might. And you especially might if you continue to go out and live life as usual."

And on ABC News' "This Week," Michigan Gov. Gretchen Whitmer pointed to a recent COVID-19 case in her state involving an infant to say the idea that "only one segment of our population that is exposed or in danger is ridiculous."

"We have to all take this seriously, and every one of us needs to do our part," she said. "Assume you are carrying COVID-19 and wash your hands, practice social distancing, stay at home if you're not absolutely needed to be outside of your home. And if you do go outside, keep that 6-foot distance from others. We all have to play our part to mitigate the spread and to save our health care system."

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Fauci said on "Face the Nation" that he does not expect the U.S.'s experience with the outbreak to turn into that of Italy, which has become one of the hardest-hit areas.

"I mean, obviously, things are unpredictable, you can't make any definitive statement, but if you look at the dynamics of the outbreak in Italy, we don't know why they are suffering so terribly, but there is a possibility that many of us believe that early on they did not shut out the input of infections that originated in China and came to other parts of the world," Fauci said before touting the Trump administration's move to restrict travel from China and then parts of Europe earlier this year.

"Again, I don't know why this is happening [in Italy] to such an extent," Fauci said. "But once you get so many of these spread out, they spread exponentially, and you can never keep up with this tsunami. And I think that's what our colleagues and, unfortunately, our dear friends in Italy are facing."

Italy was "so overwhelmed from the beginning," Fauci said, "that they can't play catch-up."

Fauci said he has "no doubt" that the virus is mutating, although experts "have not seen thus far any type of change in the way it's acting. But we are keeping a very close eye on it, because it is conceivable that it could mutate and change some of the ways that it performs."