Young people aren't immune from severe COVID-19, and a new study warns that some are more at risk than others.

Folks under 45 have more than triple the risk for severe COVID-19 if they have cancer or heart disease, or blood, neurologic or endocrine disorders, according to Mayo Clinic researchers.

"One of the surprising findings was that almost every single chronic condition category we looked at was a risk factor for more severe COVID infection," said lead study author Jennifer St. Sauver, a Rochester, Minn.-based epidemiologist.

"We also found that some of these risk factors differed across age groups. They seem more problematic in our younger population, compared to our older population," she said.

In general, younger patients tend to get less severe COVID-19 than older people, St. Sauver said. But that doesn't mean younger people aren't also becoming severely ill.

"There are younger people who are ending up in the hospital, and there are younger people who are dying," she said.

The study focused on data from nearly 10,000 U.S. patients in Minnesota and Wisconsin diagnosed with COVID-19 between March and September 2020.

Cancer was the biggest risk factor for severe COVID-19 among those under 45 but was not a significant factor for older people.

St. Sauver said younger patients tend to have more aggressive cancers and more intensive treatments, which might make them more susceptible to infections like COVID-19.

Among all those with chronic conditions, those with developmental disorders, personality disorders, schizophrenia and other psychoses had the highest adjusted risk for severe COVID-19 — a finding that took researchers by surprise.

"That was really kind of a puzzling finding, because we would accept that maybe people with severe developmental disorders or severe personality disorders or psychiatric conditions might be more likely to live in group settings and might be more exposed to COVID, but it doesn't necessarily explain why they would have a more severe condition," St. Sauver said.

One possibility is that some of the treatments to manage their conditions might put them at higher risk, she suggested.

"This is a flag for us that this is a group of people we weren't expecting who do seem to be at higher risk of more severe COVID if they get it, and that we definitely take a closer look at this population," St. Sauver said.

Some racial and ethnic groups were at greater risk than others. Asian patients had the highest risk of severe COVID-19, followed by Black and Hispanic people. Researchers said this may owe to social and economic factors that put these populations at higher risk for exposure to the virus.

St. Sauver said that vaccinations are the best way to prevent COVID-19 and everyone eligible should get vaccinated.

"We are seeing that pretty much across the board, we're incredibly lucky that there are very few contraindications to getting vaccinated, and the vaccines that we have seem to be very, very effective, even against the Delta variant," she said. "So absolutely, regardless of what you have, what your underlying health conditions are, regardless of your age, vaccination seems to be the absolutely best way to protect yourself against severe COVID."

Dr. Marc Siegel, a professor at NYU Langone Medical Center in New York City, said knowing who is at greatest risk is essential to preventing the spread of COVID-19.

"We've got to look at who is most at risk and who has chronic illnesses that don't do well with COVID," he said. "We need to risk stratify our groups for their responses to COVID — it's very important."

In risk stratification, a health risk status is assigned to groups and used to direct and improve care.

Siegel also stressed the importance of getting vaccinated.

If you're vaccinated and get COVID-19, your chance of being hospitalized is about 3%, Siegel said, adding that the vaccines are effective and safe.

He echoed recent comments from Biden administration officials, who call the current surge of COVID-19 cases an "epidemic of the unvaccinated."

"Let's protect those with chronic diseases by vaccinating them and everyone around them — please, because they're at risk for COVID complications — and the younger are not spared," Siegel said.

The study was recently published in the journal Mayo Clinic Proceedings.