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.
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.
Uptake could be low, which would be particularly worrisome for children with underlying conditions that might make them more vulnerable to being hospitalized with the virus.
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.”
After the federal government made 10 million doses available to states, pharmacies and health providers in recent weeks, the response was muted, according to data collected by federal officials. Just 2.5 million doses of Pfizer-BioNTech’s vaccine have been claimed, around half of what was available. And around 1.3 million Moderna doses, about a quarter of what was offered, had been ordered.
Federal officials have said that states typically gradually increase orders as vaccination campaigns evolve. 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., at least right away.
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.
Some pediatric and vaccine experts have said that it is a mistake to try to compare the two vaccines, in particular on the question of which is more effective.
“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 the two vaccines were studied at different times, when different variants were circulating.
He also said it was hard to pinpoint how much protection either vaccine might provide given the newer, more contagious versions of the virus that continue to mutate. “You’re kind of playing Whac-a-Mole,” he said.
Pfizer’s efficacy estimate, Dr. Moffitt said, was particularly unreliable because there were so few cases of the virus among trial participants after the third dose. “We’re going to need to wait and be patient to understand the true effectiveness of each of these vaccines.”
Pfizer’s vaccine has been authorized for children older than 5 for many months, while Moderna’s has been restricted to adults.
Dr. Moffitt and other pediatric experts said that since it had been given to millions of older children, some parents might feel more comfortable with the Pfizer-BioNTech vaccine, even though it would take longer for children to be fully vaccinated. Others, who are hoping for speedier results and might have vulnerable family members, might prefer Moderna’s vaccine.
Some families might go with the brand siblings and parents received, or take what is available nearby.
Some pediatricians, nurses and pharmacists might be more familiar with Pfizer and not want to introduce another product.
Dr. Conway said his advice to parents is that “something is better than nothing.”