Vaccines for young children are delayed by incomplete data, a top FDA official suggests.

The Food and Drug Administration has not yet approved a coronavirus vaccine for children under the age of 5 because vaccine manufacturers have not finalized their applications for permission to distribute doses, a top official at the agency suggested Tuesday.

The official – Dr. Peter Marks, who oversees vaccine regulation for the FDA – said the agency will release a schedule this week for external expert review of vaccines for the country’s 18 million children under 5. It is the only age group still not eligible for coronavirus vaccination .

Despite increasing pressure, including from Congress, the FDA may not decide whether to approve a pediatric vaccine dose for that group until June, administration officials have said.

Parents “are frustrated, they are confused, and so am I,” Senator Patty Murray, a Washington Democrat who chairs the Senate Health Committee, told Dr. Marks at an FDA regulatory hearing.

Dr. Marks said he was barred from commenting on pending requests from drug companies. But he said sharply, “Just remember, we can not actually complete our reviews until we actually have complete applications.”

Both Moderna and the partnership between Pfizer and BioNTech are developing doses of their coronavirus vaccines for very young children, but Moderna appears to be closer to submitting its request for emergency authorization. A spokeswoman for Moderna said last week that the company expects to file by the end of April, but did not say whether the company’s application would be complete by then. Pfizer is expected to submit its application in May. Many such applications involve rolling submissions of data.

In an interview Thursday with CNN +, Dr. Anthony S. Fauci, the White House’s senior medical adviser, said the FDA wanted to review data from Moderna and Pfizer at the same time to directly compare the benefits of corporate vaccines and not “confuse people.” Moderna proposes a two-shot dosing schedule for children under 6 years of age, while Pfizer is developing a three-shot vaccination with weaker individual doses for children under 5 years of age.

Dr. Fauci’s comment raised questions about whether the FDA deliberately withheld Moderna’s request to allow Pfizer to obtain it. If that is the case, then regulators would “fail millions of parents and young children,” wrote Dr. Zachary Rubin, a pediatric allergist in Illinois, on Twitter. “If they’re waiting for @pfizer to be ready, then something’s tough.”

In a letter to the FDA on Monday, the rep. James E. Clyburn, a South Carolina Democrat and chairman of a select House subcommittee on the pandemic, that the wait for Pfizer’s application could delay the approval of Moderna’s vaccine by several weeks. He asked FDA officials to brief subcommittee staff on their plans.

In response to a question at a news briefing Tuesday about the potential holdup, White House press secretary Jen Psaki said she would “refer you to the FDA for their decision-making process.”

“Of course, as a parent of a child under 5,” she added, “I’m eager to get vaccines approved and available, and I want to make sure they go through the proper protocols and process.”

Approval of a vaccine for the very young is complicated, partly because there is no agreed standard for how effective a vaccine should be against symptomatic infection. In clinical trials, conducted in part during the Omicron increase, none of the companies’ proposed vaccine performed particularly well.

Pfizer and BioNTech delayed seeking emergency clearance earlier this year after their trials showed two doses were less than 50 percent effective against symptomatic disease. The company has said it expects the three-dose plan to be more successful.

The modern vaccine also proved to be disappointing – it was only about 40 percent effective. The company is studying the effectiveness of a booster dose, and one of its top officials has said she expects a booster dose to be needed.

Alyssa Lukpat contributed with reporting.

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