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WASHINGTON — A top Food and Drug Administration official pledged Friday not to delay the rollout of coronavirus vaccines for the youngest children and said at least one of the two shots under review could become available by June.

The remarks by Peter Marks, director of the FDA’s Center for Biologics Evaluation and Research, came in an interview about the agency’s new, but tentative, timeline for handling vaccine issues during the next two months. The FDA announced plans to convene meetings with its outside advisers on June 8, 21 and 22 to consider emergency use authorizations for pediatric coronavirus shots and to hold additional sessions for other pressing vaccine matters.

Vaccine maker Moderna requested emergency use authorization for its vaccine for children 6 months to 5 years old on Thursday, and Pfizer and its German partner, BioNTech, are expected to submit the full data on a three-dose regimen for children 6 months to 4 years old in coming weeks.

Marks said the agency will not delay one vaccine to wait for the other. Each product’s application, he said, would be considered when the data are ready.

“We are not going to delay things unnecessarily here,” Marks said.

FDA officials will decide closer to the meeting dates which product will be reviewed when, depending on the status of the reviews, he said.

Marks’s promise to handle the vaccine applications expeditiously was partly a reaction to a recent story in Politico that said the FDA was considering holding off on clearing the two-dose Moderna vaccine so that it could review that product alongside the three-dose Pfizer-BioNTech vaccine.

The story suggested regulators might be uneasy about authorizing an acceptable but less-effective shot when it was possible a more potent option might be just weeks behind. Angry parents decried any potential delay involving the Moderna vaccine.

Marks said in the interview that if the vaccines are ready for consideration within a week of each other, he might present them to the agency’s vaccine advisory committee close together, on successive days. But if there is a longer lag time, he will hold separate advisers’ meetings, he said.

Marks also said that if the companies’ data hold up to scrutiny on safety, efficacy and quality, “we would anticipate June authorizations for one or more of the pediatric vaccinations.”

The FDA has come under enormous pressure to move quickly — or, at minimum, explain its plans — on pediatric vaccines.

Parents, pediatricians and politicians have expressed rising frustration that children younger than 5 remain ineligible for the shots, long after other Americans have received vaccinations and, in many cases, booster doses. Many people have sworn off masks and other protections and are attempting to revert to a pre-pandemic lifestyle, increasing concerns for unprotected unvaccinated children.

In a statement Friday, the FDA said it understands the urgency of getting vaccines to the youngest Americans but added that the vaccine makers’ submissions are not “fully complete.” The agency said that as it reviews incoming data from the companies, it will provide details on the meetings and that it would take action quickly, assuming information supports “a clear path” to authorization.

Marks said the vaccine reviews might take a few weeks longer than past emergency vaccine authorizations because of the complexity added by the variants and the need to closely examine the safety profile. Vaccine-induced fevers in young children can be more serious than in adults, he noted, causing febrile seizures.

Jeremy Faust, an emergency physician at Brigham and Women’s Hospital in Boston and a parent of a child in this age group, said the timeline should be faster than the FDA has laid out, more in keeping with the review of the Pfizer vaccine for 5- to 11-year-olds, which took less than a month.

“I agree the FDA needs to assess the side-effect profile. It would take me about six minutes. I wouldn’t need six weeks. Just show me the data,” Faust said.

The FDA also announced plans to meet June 7 with its outside advisers — the Vaccines and Related Biological Products Advisory Committee — to discuss an emergency authorization request for a coronavirus vaccine made by Novavax for people 18 and older. A vocal group of investors and some people who say they prefer the more traditional vaccine technology have been pushing for access to that vaccine for months.

Novavax submitted an application for emergency use authorization in late January. The vaccine uses a protein-based technology, with some novel twists. The vaccines from Moderna and Pfizer-BioNTech are produced using a new messenger RNA technology.

Last June, Novavax said the vaccine was 90% effective in preventing people from falling ill in a 30,000-person trial conducted when variants had begun to complicate the pandemic in the United States and Mexico. The firm has had manufacturing problems, which have delayed its vaccine.

In addition, the FDA said it will convene an advisory committee meeting June 28 to discuss whether the composition of coronavirus vaccines should be modified, and if so, which strains should be selected for use this fall. The agency and its advisers met April 6 to begin sketching the outlines of a long-term strategy for deploying vaccine booster shots against the coronavirus amid uncertainty about future variants and a rapidly closing window to prepare for a potential fall vaccination campaign.

Many experts say that additional shots for the general population should be reformulated to keep up with the changing virus, but the question of whether, when and how to do that remains unresolved. FDA officials have said a decision on the composition of a fall booster would probably need to be made by May or June.

The path to a vaccine for the youngest children has been difficult, marked by disappointing results, delays and confusing communication.

The full data package for the Moderna application is expected to be filed by early in the second week of May, according to a senior Biden administration official, who spoke on the condition of anonymity because the official was not authorized to discuss the matter publicly.

The company is seeking authorization for vaccines for children up to age 17. Children as young as 5 already have access to the Pfizer-BioNTech vaccine. Moderna’s pediatric vaccine program has been stalled for almost a year because of concerns about rare cases of heart inflammation in adolescents. Some parents have called for the agency to prioritize young children because they don’t have access to any protection.

The Pfizer-BioNTech vaccine for the youngest children had initially been expected to roll out early this year. But in December, the company announced that its two-shot regimen, which is a lower dose than Moderna’s, had failed to meet the laboratory benchmark of achieving an immune response comparable to what had been reported in young adults. The company has been testing the effects of adding a third shot.

Frustration over the pace of the pediatric vaccine intensified this week, amid concerns that a vaccine might not be available until the summer.

“We have been calling the FDA and emailing and calling our representatives” in Congress, said Katie Harmon, who has a 4-year-old unvaccinated daughter and belongs to a group of parents and pediatricians called Protect Their Future. “Kids under 5 are being left behind while others are moving on.”

The organization sent a petition, signed by more than 1,600 people, to the FDA pressing for the authorization of a vaccine as soon as safety and efficacy are established. “FDA must commit to reviewing each submission at the earliest opportunity and on an independent timeline,” the petition said.

Members of Congress, including Rep. James Clyburn, D-S.C., chairman of the select subcommittee on the coronavirus crisis, also have been pushing the FDA on pediatric coronavirus vaccines.

Harmon said she would get her daughter vaccinated immediately, even if it means using a two-dose Moderna vaccine that might not be as effective as a three-dose Pfizer-BioNTech regimen.

“I feel the best shots are the ones you can get right away,” Harmon said.

But Brooke Watts, who lives in Tacoma, Wash., has daughters ages 1 and 4 and said she thinks it would be fine for the FDA to wait a little longer and deal with both vaccines at the same time.

She said that the number of COVID-19 cases in her area is fairly low and that her pediatrician said the chance of her daughters developing serious illness from the coronavirus is small.