FDA’s vaccine chief Dr. Peter Marks said he hoped parents would understand that the decision to delay was part of the agency’s careful review and high scientific standards.

“We take our responsibility for reviewing these vaccines very seriously because we’re parents as well,” Marks told reporters during a teleconference.

Pfizer’s early data showed two of the extra-low doses were safe for kids under 5 and strong enough to give good protection to babies as young as 6 months. But once tots reached the preschool age — the 2- to 4-year-olds — two shots didn’t rev up enough immunity.

And a study of a third dose isn’t finished yet — meaning the FDA was considering whether to authorize two shots for now with potentially a third cleared later, something highly unusual.

COVID booster effectiveness wanes but remains strong, study shows

Friday, the FDA didn’t say exactly what new data Pfizer was providing except that it involved the critical issue of a third dose.

“We believe additional information regarding evaluation of a third dose should be considered as part of our decision-making,” the agency said in a statement.

The nation’s 18 million children under 5 are the only age group not yet eligible for vaccination.