The head of the second-largest teachers union on Sunday said that COVID-19 vaccinations should be required for teachers to protect students who can not be vaccinated. The announcement comes as more children are becoming infected during a surge that has hit many southern states.

"The circumstances have changed," Randi Weingarten, president of the American Federation of Teachers, told NBC News' "Meet the Press."

"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," she said.

Health experts are saying that the Delta variant is more likely to infect children than the original strain.

Dr. Anthony Fauci, the nation's top infectious disease official, has stressed that it is important to surround children with vaccinated and masked people until shots can be approved for younger ages.

​​"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," he said in a separate interview on NBC News.

The Biden administration has said that almost 90% of educators are vaccinated but the number of hospitalized children continues to rise.

Former Food and Drug Administration Commissioner Dr. Scott Gottlieb said that not requiring masks for students as they return to full-day, in-person learning was reckless.

Florida Gov. Ron DeSantis recently refused to mandate masks in schools and has blocked the school districts from doing so, despite cases hitting record numbers this week.