Amid increased fears that children are now both victims and vectors of the latest Covid-19 variant surge, National Institutes of Health director Francis Collins signaled on Sunday that increasing numbers of children are falling ill in the US.

His comments also came as one of America’s largest teachers unions appeared to shift its position on mandatory vaccinations for teachers.

With around 90 million adult Americans remaining unvaccinated, and vaccines remaining unauthorized for 12 years and under, Collins told ABC News This Week with George Stephanopoulos that “the largest number of children so far in the whole pandemic right now are in the hospital, 1,450 kids in the hospital from Covid-19.”

Collins acknowledged that data on pediatric infections was incomplete but he said that he was “hearing from pediatricians that they’re concerned that, this time, the kids who are in the hospital are both more numerous and more seriously ill”.

Collins’s comments came as new Covid-19 cases in the US have rebounded to more than 100,000 a day on average, returning to the levels of the winter surge six months ago. But health officials focus on children adds urgency to the situation as the US education system approaches the start of the school year.

Collins said his advice to parents of school-age children is to “think about masks in the way that they ought to be thought about”.

He added: “This is not a political statement or an invasion of your liberties. This is a life-saving medical device. And asking kids to wear a mask is uncomfortable, but, you know, kids are pretty resilient. We know that kids under 12 are likely to get infected and if we don’t have masks in schools, this virus will spread more widely.”

The alternative, he said, “will probably result in outbreaks in schools and kids will have to go back to remote learning which is the one thing we really want to prevent”.

Warning that virtual learning that kids have experienced for more than a year is “really bad for their development”, Collins urged that “we ought to be making every effort to make sure they can be back in the classroom. And the best way to do that is to be sure that masks are worn by the students, by the staff, by everybody.”

The president of the American Federation of Teachers, Randi Weingarten, went further, calling for vaccine mandates for teachers. “As a matter of personal conscience, I think that we need to be working with our employers, not opposing them, on vaccine mandates,” she told NBC’s Meet The Press.

Weingarten’s comments are an advance on the union’s earlier position in which it maintained teachers should be prioritized for the vaccines but stopped short of supporting a mandate. That shift was previewed last week when Weingarten said she would consider supporting vaccine mandates to keep students and staff safe and schools open.

Dr Anthony Fauci, chief medical adviser to Joe Biden, also echoed Weingarten’s comments Sunday, saying the best way to protect children from the virus is to “surround them with those who can be vaccinated, whoever they are. Teachers, personnel in the school, anyone, get them vaccinated.”

Dr Scott Gottlieb, former FDA commissioner, also weighed in on the concerns, saying that schools are not “inherently safe” from the Delta variant and that society “can’t expect the same outcome that we saw earlier with respect to the schools where we were largely able to control large outbreaks in the schools with a different set of behaviors.”

“The challenge right now is that the infection is going to start to collide with the opening of school. And we have seen that the schools can become sources of community transmission when you’re dealing with more transmissible strains,” Gottleib told CBS’s Face the Nation.