(WHTM) — The FDA says it is working around the clock to make COVID-19 vaccines available to younger children as millions return to classrooms and pediatric cases surge across the country.

Los Angeles, the nation’s second-largest school district, is requiring shots for public school students 12 and up. For those younger than 12, the FDA urges parents not to get them vaccinated before the FDA signs off, saying in part, “children are not small adults.”

“It’s never been this crazy, never been this busy. It’s exhausting all of us. And our biggest concern, of course, sick kids aren’t getting the care they need,” Dr. Bryan Kornreich, a pediatrician in Virginia, said.

“We don’t know the right dosing for children under 12. It may be different. We haven’t seen the data on safety and effectiveness in kids under 12,” Dr. Ashish Jha, Brown University School of Public Health’s dean, said.

The department of health and human services says more than 2,500 children are currently hospitalized with COVID-19, nationwide.