The Food and Drug Administration on Friday laid out a tentative timetable for deciding whether to authorize a coronavirus vaccine for America’s youngest children, announcing that June 8 is the earliest date that it will present data to its outside advisers for a recommendation.

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

The agency said it understood the urgency of protecting that group and that it would act quickly “if the data support a clear path forward following our evaluation.”

The F.D.A. laid out a packed June schedule, setting aside three days for its outside advisory committee to consider the pediatric vaccine applications from Moderna and Pfizer-BioNTech, and another day to debate and presumably vote on proposals to update existing vaccines so they work better against a range of coronavirus variants. The dates are tentative, the agency said, because the companies have not yet completed their applications for emergency authorization.

The agency’s release of a schedule came after some critics raised concerns that it was delaying action on Moderna’s application in order to compare it with data from Pfizer, which is not expected in full until June. The F.D.A. commissioner, Robert Califf, said that was not the case in interviews on Thursday and Friday. “Categorically, there is no reason for the F.D.A. to wait,” he told Andy Slavitt on the podcast “In the Bubble.”

Moderna said Thursday that it had asked the F.D.A. to authorize its vaccine for children six months to 6 years old, saying its clinical trial showed the vaccine was safe and produced an antibody response comparable to that of adults ages 18 to 25. That met the trial’s criterion for success. It said 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 efficacy rates are far below those of earlier clinical trials conducted by both Moderna and Pfizer before the highly transmissible Omicron variant emerged. Omicron and its subvariants have proven far more adept at evading protection from antibodies that constitute the immune system’s first line of defense against infection.

Because there were no cases of serious illness or hospitalization among any of the trial participants, it was not possible to judge how well the vaccine protects against Covid’s worst outcomes.

Moderna said it would finish submitting its data to the F.D.A. by May 9. Pfizer and BioNTech are expected to complete their application to vaccinate children under 5 in June. The vaccine regimens are different: Moderna is proposing two doses, 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.

Pfizer’s vaccine is already authorized for everyone 5 years and older, while Moderna’s shots are limited to adults. Moderna has previously requested authorization of its vaccine for children between the ages of 6 and 17. The company said it plans to submit data supporting and updating those requests in about two weeks.

The F.D.A. appears to want to consider Moderna’s applications to vaccinate several age groups at the same time. Dr. Peter Marks, who oversees vaccine regulation, testified at a Senate hearing this week that it takes more time to review applications that cover a broader swath of the population.

If so, that stance could trigger criticism. “That would certainly make the messaging somewhat easier: ‘Here’s this vaccine. It’s now available for all kids,’” Dr. Jesse L. Goodman, a former chief scientist with the F.D.A., said in an interview Thursday.

But, he added: “The Pfizer vaccine is available for the older children. This younger group right now is the major unmet need. I don’t think there would be something wrong with authorizing the vaccine for the younger ones first, if they were ready.”

The agency set June 28 for the advisory committee to consider the issue of how best to update the existing vaccines. Researchers working for the National Institutes of Health and vaccine manufacturers have been racing to try to redesign the vaccines so they are more effective against newer virus variants. Federal health officials have said they need to choose one or more revised versions by June in order to manufacture doses by the fall, when they expect the coronavirus to resurge.

The agency also said that it plans to present vaccine data on June 7 from Novavax, which has developed a vaccine using a more conventional approach than the Pfizer-BioNTech and Moderna shots that have now been used to inoculate hundreds of millions of Americans.

Those vaccines are based on newer messenger RNA technology, while Novavax’s vaccine is protein-based. Protein-based vaccines have been used for decades and generally have a strong track record of safety and mild side effects.

If authorized, it is not clear what role Novavax vaccine would play in the nation’s vaccination program. But some experts suggest that people who are hesitant to take the Moderna or Pfizer-BioNTech shots might be more willing to accept the more conventional Novavax doses. It is also possible that a protein-based vaccine might work well as a booster to Pfizer or Moderna shots.