A United States decision on Pfizer and BioNTech’s COVID-19 vaccine for children six months through four years of age has been postponed for at least two months after the Food and Drug Administration (FDA) said it needed more data.

The FDA had planned to make a decision on the vaccine based on early trial data because of what it had called a great public health need due to the surge in infections caused by the Omicron variant of the coronavirus. The decision was slated for next week, with a rollout starting as soon as February 21.

On Friday, the agency said it had reviewed new trial information that had come in since Pfizer and BioNTech’s request for emergency authorisation and decided it needed more data before moving forward.

The FDA said parents anxiously awaiting the vaccine for the roughly 18 million children in the age group should be reassured that the agency is taking the time to make sure it meets the standard it has set for authorisation.

“If something does not meet that standard, we can’t proceed forward,” said Dr Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research. Marks noted some of the new data that pushed the FDA to delay the decision was “late-breaking”.

Pfizer and BioNTech had submitted data on the first two doses of what was planned as a three-dose regimen for this age group earlier this month at the request of the FDA; they did not disclose efficacy data.

The submission was surprising because in December they said initial trial results of the lower-dose vaccine fell short of expectations in two- to four-year-olds and amended their clinical trial to test a three-dose version.

The companies said they would continue the trial to dose all children with three shots and expected to have data in April.

“This is a three-dose vaccine, and they were going to be presenting data on the first two doses. It makes sense to wait for the safety and efficacy data on all three doses to be available before we make a decision about this vaccine,” said Dr Paul Offit from Children’s Hospital of Philadelphia.

Offit is a member of the FDA’s Vaccines and Related Biological Products Advisory Committee that had been scheduled to vote on whether to recommend authorisation of the shot for kids under five on Tuesday. The meeting was postponed.

“I’m not sure where this all came from. Why were we being asked to do this?” he said.

The primary series of the Pfizer/BioNTech vaccine has been two doses in all older age groups. But in December, Pfizer changed the design of its clinical trial to test a third dose of the vaccine in the age group because the lower dose generated an immune response in two- to four-year-olds that was inferior to the response measured in those aged 16 to 25 in previous clinical trials.

In six- to 24-month-old children, the vaccine generated an immune response in line with 16- to 25-year-olds.

The delay may be disappointing for harried parents of younger children who have had to contend with quarantines and closures of preschools and daycare centres.

Still, Dr Amesh Adalja, an infectious disease expert at the Johns Hopkins Center for Health Security, said there was considerable pushback about the FDA’s decision to pursue authorisation so quickly, “as this age group is very low risk for severe disease and vaccine uptake in the five-11 [age group] has been very suboptimal”.

“It’s critical that people have confidence in the process if higher vaccine uptake is the goal,” Adalja said.