As the battle against coronavirus continues in the United States, new government data suggests that every American, old and young, is at risk of severe illness.

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Nearly 4 in every 10 cases requiring hospitalization involved people under the age of 55, according to a report from the US Centers for Disease Control and Prevention.

Researchers examined outcomes for the 4,226 cases of non-imported COVID-19 reported in the United States as of March 16.

Similar to earlier data out of China, 80% of people who died were at or above 65, "with the highest percentage of severe outcomes among persons aged 85 years [or older]," wrote the CDC's COVID-19 Response Team.

That's not unexpected, since it's long been understood that age—and the chronic health issues that often accompany it—are major risk factors for fatalities from COVID-19.

But the study also found a higher-than-expected rate of hospitalization, including admission to the ICU, among much younger patients.

For example, among hospitalized cases, 38% were aged between 20 and 54, the report found. And among those most critical cases—those requiring admission to an ICU—12% were for patients ages 20 to 44, and 36% were for patients between 45 and 64, the CDC team said.

Less than 1% of hospitalized patients were under the age of 19, the report found, and there were no deaths in that age group.

Why the unexpectedly high number of serious cases in younger adults? The answer isn't yet clear, the researchers said, and they stress that "no data on serious underlying health conditions were available." So, younger people with certain health issues might be at greater risk—but data to confirm that isn't yet available.

Speaking at a White House briefing earlier this week, virologist and coronavirus task force coordinator Dr. Deborah Birx stressed that "you could be 40 and have a significant medical condition and be at substantial risk. You could be 30 and having come through Hodgkin disease, or non-Hodgkin lymphoma, and be at significant risk. So, there are risk groups in every age group."

Certainly the early findings published Wednesday "demonstrate that severe illness leading to hospitalization, including ICU admission and death, can occur in adults of any age with COVID-19," the CDC authors said.

"I think everyone should be paying attention to this," Stephen Morse, a professor of epidemiology at Columbia University's Mailman School of Public Health, told The New York Times. "It's not just going to be the elderly. There will be people age 20 and up. They do have to be careful, even if they think that they're young and healthy."

The finding also hints that if so many younger adults are ending up in hospitals due to coronavirus infection, that may only be the tip of the iceberg—many thousands more may have asymptomatic or minimally symptomatic illness.

Dr. Christopher Carlsten is head of respiratory medicine at the University of British Columbia in Canada. Speaking with the Times, he said that "younger people may feel more confident about their ability to withstand a virus like this. [But] if that many younger people are being hospitalized, that means that there are a lot of young people in the community that are walking around with the infection."

And that formed the crux of a plea earlier this week to young American adults from Birx.

"Millennials are the key, because they are the ones that are out and about, and they are the ones that are most likely to be in social gatherings, and they are the ones that are most likely to be asymptomatic," she told reporters at the Monday White House briefing.

"The millennials can help us tremendously," Birx added. "Millennials can speak to one another about how important it is in this moment to protect all of the people."

As of Thursday, total US cases of coronavirus infection passed 9,000, with 149 deaths recorded. As of Tuesday, the WHO had reported nearly 208,000 cases of coronavirus across 166 countries and territories, including over 8,600 deaths.