Moderna’s clinical trial data showed that the antibody response of the youngest children compared favorably with that of adults aged 18 to 25.Credit...Rick West/Daily Herald, via Associated Press

Moderna said Thursday that it had asked the Food and Drug Administration to authorize its coronavirus vaccine for children under 6, making it the first manufacturer to do so. A top official at the company said it would finish submitting data to regulators by May 9.

The development, which was expected, steps up pressure on federal regulators to authorize a vaccine for the nation’s youngest children. Parents of the roughly 18 million youngest Americans, the only population group not yet eligible for vaccination, have been increasingly vocal about their frustration with delays, and members of Congress have been asking the F.D.A. why it can’t move faster.

Moderna’s application for emergency use authorization is complicated because so far, its vaccine has been cleared only for adults. The F.D.A. has generally authorized Covid vaccines according to age group, calculating that the risks of severe disease and the benefits of vaccination are highest for the oldest Americans and reduced for young recipients.

Moderna has already requested authorization of its Covid vaccine for 6- to 11-year-olds and 12- to 17-year-olds, and a spokeswoman said that it would submit data supporting and updating those requests in about two weeks.

At the moment, the F.D.A. appears to be leaning toward considering all three of the company’s applications to vaccinate children simultaneously. At a Senate oversight hearing on Tuesday, Dr. Peter Marks, who oversees vaccine regulation for the F.D.A., said some requests “are complicated because they are relatively larger, covering larger swaths of the pediatric population than others,” an apparent reference to Moderna.

The F.D.A. has promised to ask its outside advisory panel of experts to review the data before the agency authorizes any vaccine for the youngest children. Meetings of the panel that were tentatively scheduled for May have been moved to June, suggesting that no regulatory decision will come next month. The agency is expected to release a more definitive timetable for the advisory panel meetings as soon as Friday.

Outside experts said an expedited review of Moderna’s application could take until June. By then, Pfizer and BioNTech, its German partner, may have completed their own application for pediatric doses covering children 6 months to 4 years old.

The companies’ vaccines are different in key respects. Moderna is proposing a two-dose regimen for children less than 6 years old, using one-fourth the strength of an adult dose. Pfizer and BioNTech are working on a three-dose regimen, at one-tenth the strength of the adult dose. The Pfizer-BioNTech vaccine is already authorized for children 5 and older, giving the companies a track record with pediatric doses.

In an interview last week, Dr. Anthony S. Fauci, the chief medical adviser to the White House, suggested that the F.D.A. wants to review Moderna and Pfizer’s data for the youngest age group simultaneously in order to directly compare the merits and not “confuse people.” That prompted questions from some pediatricians about whether the agency would delay action on Moderna’s application while waiting for Pfizer’s data.

But outside experts said they would be surprised to see the F.D.A. take such a stance. “There is some discretion there, but if you have something, you’ve reviewed it and it’s meeting an unmet medical need, you’re going to want to get it out there when you can,” Dr. Jesse L. Goodman, a former chief scientist for the agency, said.

Dr. Paul Burton, Moderna’s chief medical officer, said in an interview Wednesday that “the data that we have now, that will go in today, should be sufficient for F.D.A. to begin the review.” He said he hoped that the agency would “carefully and appropriately assess the data and not hold it up,” adding: “They’ll do the right thing. They always have.”

Moderna’s clinical trial data showed that the antibody response of the youngest children compared favorably with that of adults ages 18 to 25, meeting the trial’s primary criterion for success. Although the trial was not big enough to measure vaccine effectiveness, Moderna said Thursday the vaccine appeared to be 51 percent effective against symptomatic infection among those younger than 2, and 37 percent effective among those 2 to 5.

Those results were slightly better than the ones Moderna previously released for children under 2. The company said that was because the second time, the firm excluded infections that had not been confirmed with a P.C.R. test analyzed in a laboratory. Dr. Burton said the new results came from a more rigorous analysis, but did not differ significantly from the earlier ones.

He said the Omicron variant accounted for about 80 percent of infections in the study group. Side effects were at a similar level as those from previously approved pediatric vaccines, with fevers in 15 percent to 17 percent of the children, Moderna said.

Omicron has scrambled the calculus for evaluating vaccines because it has proved far more adept than previous versions of the virus at evading the vaccines’ shield against infection, although their protection remains strong against severe illness and death. Both Moderna and Pfizer found that compared with earlier trials, their vaccines’ effectiveness against infection plunged in clinical trials for young children, which were carried out largely during the winter Omicron surge. There is no agreed-upon standard for what would constitute a successful Covid vaccine for young children.

Even while it limited the vaccines’ protection, Omicron helped build up the nation’s immunity. As of February, 60 percent of Americans, including 75 percent of children, had been infected with the coronavirus at some point, according to research released this week by the Centers for Disease Control and Prevention. In December 2021, as the Omicron variant began spreading, only half as many people had antibodies indicating prior infection, the agency said.

Dr. Sean O’Leary, an infectious disease specialist at Children’s Hospital Colorado, said the C.D.C.’s findings did not lessen the need to vaccinate young children. “A lot of kids have been infected,” he said, but “there is certainly additional benefit of getting vaccinated.” He added: “I also recognize that it’s a complex situation” for the F.D.A.

The fact that the pandemic is in a relative lull may give the agency a little breathing room. For those 17 and under, the rate of new hospital admissions is one-eighth of what it was in January, when it hit a high. Daily admissions now number fewer than 120 children, out of about 73 million nationwide, according to the C.D.C.