COVID-19 vaccination should be made mandatory for teachers in the U.S. to protect students who are too young to be inoculated, the head of the nation's second-largest teachers' union said on Sunday, shifting course to back compulsory shots as more children fall ill.

"The circumstances have changed," Randi Weingarten, president of the American Federation of Teachers, told NBC News' Meet the Press program. "It weighs really heavily on me that kids under 12 can't get vaccinated."

"I felt the need to stand up and say this as a matter of personal conscience," said Weingarten.

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COVID-19 in U.S.: Over 4m children get virus, weekly cases go up again

The number of children hospitalized with COVID-19 is rising across the country, a trend health experts attribute to the Delta variant being more likely to infect children than the original Alpha strain.

With around 90 million adult Americans remaining unvaccinated and vaccines remaining unauthorized for 12 years and under, National Institutes of Health Director Francis Collins told ABC News This Week with George Stephanopoulos that "the largest number of children so far in the whole pandemic right now is in the hospital, 1,450 kids in the hospital from COVID-19."

The White House is ramping up efforts to boost COVID-19 vaccinations among ​U.S. children as young as 12 and young adults, ahead of their return to school this fall.

Almost 90 percent of educators and school staff are vaccinated, according to a White House statement.

Dr. Anthony Fauci, the nation's top infectious disease official, said it was critical to surround children with vaccinated and masked people in schools and elsewhere until shots are approved for them.

"You surround them with those who can be vaccinated, whoever they are – teachers, personnel in the schools, anyone – get them vaccinated. Protect the kids with a shield of vaccinated people," Fauci said in a separate interview on NBC, noting that pediatric hospitals are filling up with COVID-19 cases.

(With input from Reuters)