Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases, testifies during a Senate Health, Education, Labor and Pensions Committee hearing on the federal coronavirus response on Capitol Hill in Washington, Thursday, March 18, 2021. Credit: Susan Walsh / AP

 The omicron variant of COVID-19 shows signs of causing milder illness but the “really unprecedented” rise in cases is still likely to cause serious sickness in many unvaccinated Americans, Anthony Fauci said.

“When you have so, so many cases, even if the rate of hospitalization is lower with omicron than it is with delta, there’s still the danger that you’re going to have a surge in hospitalizations that might stress the health care system,” U.S. President Joe Biden’s chief medical adviser said on CNN’s “State of the Union.” “We’re still going to get a lot of hospitalizations.”

Fauci was referring to the delta variant of the coronavirus, which caused a deadly wave of infections around the U.S. last year.

Now hospitalizations are rising again, and average daily infections are at a record 400,000, Fauci said on ABC’s “This Week.” Several states have mobilized their National Guards to help provide medical care amid staffing shortages and, like Texas, asked for federal help.

Almost 73 percent of U.S. adults are fully vaccinated, and 36.3 percent of adults have received a booster shot, according to data from the U.S. Centers for Disease Control and Prevention.

Fauci said that despite omicron’s rapid spread, schools should remain open, though with masking and other rules, and he urged parents to get eligible children vaccinated.

Mitigation strategies including masking and vaccines are working in schools, which should remain open if possible, U.S. Education Secretary Miguel Cardona said Sunday.

“Our default should be in-person learning,” Cardona said on CBS’ “Face the Nation,” citing the benefits of classroom education over virtual learning.

Ian Fisher and Brendan Case, Bloomberg News