There’s growing evidence that the COVID-19 delta variant is causing more serious illness in children, Dr. Francis Collins, director of the National Institutes of Health, said Tuesday. Collins, however, conceded that U.S. health officials don’t yet have concrete data on the matter.

In an interview with CNN’s “New Day,” Collins cited reports out of hospitals in Louisiana and other hot-spot states about increasing numbers of children being hospitalized with COVID-19, as well as studies out of Singapore, Scotland and Canada that found the delta variant “is not just more contagious, but more serious in various age groups.”

Collins said children are still less likely to become sick from the coronavirus than adults, “but anybody who tries to tell you, you don't have to worry about it if you're a young, healthy person, there's many counter examples all around us now.”

“We don't have really enough numbers to be confident, but it certainly tilts the balance in that direction,” Collins added. 

Children under 12 years old are not yet eligible to be vaccinated. Collins said they should avoid going to places where they might get infected and that masks should be worn in schools, as the Centers for Disease Control and Prevention recommended last week.

Some hospitals in Louisiana, Arkansas, Alabama and Florida have reported seeing record numbers of children being hospitalized with COVID-19, many of them being placed in intensive care. The growing problem appears to be almost entirely among unvaccinated children, according to various news reports.

In Louisiana, the state with the most new COVID-19 infections per-capita among all age groups, children accounted for more than 2,000 of 11,000 new cases this past weekend, Gov. John Bel Edwards said.

Dr. Catherine O'Neal, chief medical officer at Our Lady of Lake Regional Medical Center in Baton Rouge, told CNN that in June her hospital would see four to six children a week in the emergency room with COVID-19, with very few being admitted. Now, they are seeing 40 to 60 a week in the E.R., with more and more being admitted.