(NEXSTAR) — Federal health regulators on Friday delayed next week’s public meeting to review Pfizer and its partner BioNTech’s COVID-19 vaccine for children under 5, saying they want to see more data.

The surprise announcement by the Food and Drug Administration raises questions about how soon youngsters could get the shots. The expert panel had been scheduled to meet Tuesday to determine if children under 5 should start getting extra-low doses of Pfizer’s vaccine before it’s clear if they’ll need two shots or three.

In an extraordinary move earlier this year, the FDA had urged the companies to apply earlier than they had planned.

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.

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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.

Also on Friday, Pfizer announced that the company will push back its submission to the FDA. Pfizer, which has been sharing trial data with the FDA on an ongoing basis, said it has decided to wait until data from the three-dose trial is complete, which the company expects to have in early April.

“The extension allows the FDA time to receive updated data on the two and three-dose regimen, conduct a thorough evaluation of it and facilitate a robust, public discussion,” Pfizer said in a news release.

Under the previous projected timeline, young children might have been cleared to receive the vaccine in early March.

Pfizer said the trial has been proceeding rapidly, in part thanks to a surge of data driven by the wave of omicron-variant infections among children under 5 years of age.

“Given that the study is advancing at a rapid pace, the companies will wait for the three-dose data as Pfizer and BioNTech continue to believe it may provide a higher level of protection in this age group,” the release says. “This is also supported by recent observations of three dose booster data in several other age groups that seems to meaningfully augment neutralizing antibody levels and real world vaccine protection for omicron compared to the two-dose regimen.”

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