The Food and Drug Administration authorized on Friday both the Moderna and Pfizer-BioNTech coronavirus vaccines for the nation’s youngest children, a relief to many parents anxious to protect their children 18 months after shots were first made available to adults.

Regulators followed the recommendation of independent experts on an advisory panel, who voted unanimously on Wednesday in favor of both vaccines. All Americans other than the roughly 20 million children under the age of 5 have had the option of protecting themselves against Covid-19 for months and are eligible for booster shots.

Moderna’s two-dose vaccine is for children 6 months through 5 years old, while Pfizer’s three-dose vaccine is for children 6 months through 4 years old.

A separate advisory panel to the Centers for Disease Control and Prevention is meeting Friday and Saturday on the matter. Afterward, the C.D.C.’s director, Rochelle P. Walensky, will issue her own recommendation, the final step before doses are rolled out, probably early next week.

The Biden administration will begin shipping the vaccines on Friday. But uptake in the coming weeks could be low, which could be particularly worrisome for children with underlying conditions that can increase the risks of severe disease.

Both Pfizer’s and Moderna’s studies showed that the Omicron variant of the virus greatly dulled the power of the vaccines to prevent symptomatic infection in young children, just as it did for adults.

Pfizer has said that two doses of its vaccine were only about 28 percent effective in preventing disease, while three doses were 80 percent effective. But that 80 percent estimate was based on only 10 cases in a subset of 1,678 trial participants.

Moderna’s vaccine was about 51 percent effective in children 6 months to 2 years old and 37 percent effective in children 2 through 5.

Still, Dr. Robert M. Califf, the F.D.A.’s commissioner, said in a statement, “As we have seen with older age groups, we expect that the vaccines for younger children will provide protection from the most severe outcomes of Covid-19, such as hospitalization and death.”

Both vaccines provoked an immune response comparable to that triggered in young adults, the primary criteria by which the agency measured effectiveness. Asked to compare them at a press briefing, Dr. Peter Marks, who heads the F.D.A.’s vaccine division, said that Moderna’s vaccine might trigger an immune response “slightly more rapidly,” while Pfizer’s might “bring a greater response after the third dose.”

But he said parents should feel comfortable with either, and he recommended they go with whichever one is available.

“Today is a day of huge relief for parents and families across America,” President Biden said in a statement.

Asked why the F.D.A.’s statement on the authorizations cited efficacy data only for Moderna’s shots, Dr. Marks said the company’s data was “more mature” than Pfizer’s. Another top agency regulator on Wednesday said that Pfizer’s estimate of 80 percent effectiveness for three doses was “imprecise” and could change.

Federal agencies are now studying whether both vaccines need to be updated for all age groups to counter the rapidly evolving virus. But Dr. Marks urged parents not to wait to see if the vaccines are redesigned, saying that the current ones still provide “excellent foundational immunity against a broad range of Covid-19 variants.”

Early indications suggest there may be a big surplus of the pediatric doses. So far, only 2.5 million doses of Pfizer-BioNTech’s vaccine have been ordered, around half of what the federal government offered states, pharmacies and health providers. And about 1.3 million Moderna doses, about a quarter of what was offered, had been claimed.

Cori J. Darr, of Riverside, Mo., said early Friday morning that she would seek the vaccine immediately for her daughter, who will turn 5 in November. “I was starting to think that she would age into a vaccine before anyone bothered to approve one,” she wrote in an email.

But she said so far she is having trouble figuring out where to go. She said neither her pediatrician’s office nor her local health department plans to offer the shot, and the vaccine clinic at her local children’s hospital has closed.

Federal officials have said that states typically gradually increase their orders as vaccination campaigns go on. But less than 40 percent of children ages 5 to 11 have received at least one dose of a coronavirus vaccine, a sign that families, including those whose adult members were eagerly vaccinated, might be hesitant to inoculate their children.

Pfizer’s vaccine regimen is three doses at one-tenth the strength of its adult dose. The first two doses are three weeks apart; the third dose is at least two months after the second. That means it would take three months to be fully vaccinated.

Moderna’s vaccine is two doses, four weeks apart at one-fourth the strength of its adult dose. Its dose is significantly stronger dose than Pfizer’s and the time frame is shorter, which might appeal to parents eager for children to finish the series.

The F.D.A. has said both vaccines might need boosters. That would mean that Pfizer-BioNTech’s vaccine would be four doses and Moderna’s would be three.

Dr. Erin Rose Medina, of Spokane, Wash., said she prefers Moderna for her three-year-old son because she thinks it may be more effective. “Another reason to go with Moderna — faster to get to fully vaccinated status,” she wrote in an email. “If only Pfizer is available we won’t wait for Moderna,” she added.

Some pediatric and vaccine experts have said that it is extremely hard to try to compare the two vaccines.

“I don’t think there is convincing enough data to say one is better than the other,” said Dr. Kristin Moffitt, a physician at Boston Children’s Hospital who has treated young Covid-19 patients. “We all need to acknowledge that to compare the two head-to-head is very tricky, given that they were studied differently.”

Dr. James Conway, a pediatric infectious disease expert at the University of Wisconsin-Madison. said it was hard to pinpoint how much protection either vaccine might provide given that they were tested against a variant has been superseded. “You’re kind of playing Whac-a-Mole,” he said.

Experts said some parents, pharmacists and pediatricians might feel more comfortable with Pfizer’s vaccine simply because it has been authorized for children 5 and older for many months. Until Friday morning, Moderna’s vaccine was restricted to adults. The F.D.A. authorized it for three age groups, from 6 months through 17, simultaneously.

The F.D.A. has faced some criticism for not authorizing Moderna’s vaccines for children under 6 earlier, given the unmet need. According to Moderna, the firm completed its application for all three pediatric age groups on May 9.

At the press briefing, Dr. Marks insisted that regulators did not delay action on Moderna’s request. He added that clearing both vaccines for the youngest children “actually gives people choice.”

Emily Erdos contributed reporting.

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