COVID-19 vaccinations should be required for U.S. teachers 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 mandated 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."