“Many parents, caregivers and clinicians have been waiting for a vaccine for younger children and this action will help protect those down to 6 months of age,” FDA Commissioner Robert Califf 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.”

In addition to authorizing the mRNA vaccines for the youngest children, the FDA authorized Moderna’s Covid-19 vaccine for children 6 to 17. For all ages, the FDA determined that a third dose of Moderna’s shot should be given to children with certain types of compromised immune systems at least one month after they receive their second shot. The Pfizer-BioNTech vaccine had already been authorized for children as young as 5.

The CDC’s panel of expert advisers will consider whether to recommend the shots’ administration during meetings on Friday and Saturday. Once CDC Director Rochelle Walensky signs off on a recommendation, children are expected to begin receiving shots by Tuesday.

Children under 6 who receive the Moderna vaccine will get two 25-microgram doses four weeks apart. The Pfizer vaccine is two 3-microgram doses three weeks apart, followed by a third dose at least eight weeks later.

Some members of the FDA advisory panel signaled concern that parents may get confused by the products’ different dose regimens — particularly since the Pfizer-BioNTech vaccine doesn’t offer much protection after two doses, while Moderna’s primary series is complete with two doses.

“I have a lot of concern that many of these kids will not get the third dose,” Jeannette Yen Lee, a biostatistics professor at the University of Arkansas for Medical Sciences, said of the Pfizer vaccine. “It’s a struggle to get people in for two,” she added, noting that booster uptake for older populations is also low.

Real-world efficacy against the Omicron variant in the 6-month-to-5-year-old age group for Moderna’s vaccine ranged from 36 percent to 51 percent, and efficacy estimates were “generally consistent” with rates seen in observational studies of adults during the same variant waves, the FDA said.

Preliminary analyses of the Pfizer vaccine showed efficacy of 80 percent in kids under 5 against disease, though only 10 Covid cases were reported among study participants before the data cutoff date in April, limiting confidence in that figure.

Some FDA advisers expressed concern that parents will compare efficacy percentages put forward by the companies and base which product they pick solely on those numbers. When asked in a press conference, Comissioner Califf said that speed should be the decision maker for parents. “I’m a grandparent, and I’ve got two that are in the age category we’re discussing,” Califf said in a press conference with reporters. “They all get the first one that’s available.”

To date, states, territories, pharmacies and other federal partners have ordered approximately 2.5 million doses of the Pfizer vaccine — half of what’s been offered so far — and 1.3 million doses of Moderna, or about a quarter of what’s been made available for pre-order, Assistant HHS Secretary for Preparedness and Response Dawn O’Connell said Thursday.

Michael Nelson, chief of UVA Health’s asthma, allergy and immunology division, urged the manufacturers to quickly gather data on the prospect of vaccinating these children against Covid at the same time they receive other routine childhood immunizations.

“If we don’t get a quick answer to the coadministration question, it will serve as a barrier to completion of the three-dose series for [the Pfizer] vaccine and likely for the Moderna vaccine,” he said. “Having to get it in isolation is going to be a great challenge to families and children here in the U.S.”

“Today is a day of huge relief for parents and families across America,” said President Joe Biden in a statement. “As early as next week, pending recommendations from the Centers for Disease Control and Prevention, parents will finally be able to get their youngest kids the protection of a safe and effective Covid-19 vaccine.”

The Biden administration is girding for a slog in convincing parents to quickly vaccinate their young children. Summer vacations — and young children receiving various levels of schooling before age 5 — along with misinformation about vaccines could depress early turnout. Many young children also contracted Covid during the Omicron surge, which could convince parents to hold off on immunizing them until they’re further removed from their natural infections.

The administration has faced criticism for the time it took to authorize these vaccines. Moderna asked the FDA for authorization in April, well before Pfizer-BioNTech asked the FDA to authorize their vaccine earlier this month. Moderna shots could have conceivably been ready to go into arms and thighs by mid-May.

“I wouldn’t call this a delay,” said Peter Marks, the director of the Center for Biologics Evaluation and Research. “There was an entire submission from 6 months through 17 years of age. It was very important to get [the data review] correct, and we did.”

“It so happens that toward the end, we converged with the updated submission from Pfizer,” he added. “We believe that actually bringing these together to the American public ... actually gives people choice.”

Recent polling by the Kaiser Family Foundation suggests about 20 percent of parents are eager to vaccinate their children under 5 as soon as they’re allowed, while nearly 40 percent plan to “wait and see” how the vaccine works and another 40 percent are reluctant to immunize at all.

Just 29 percent of U.S. children ages 5 to 11 are fully vaccinated against Covid, compared to nearly 56 percent of 12- to 15-year-olds and 67 percent of 16- and 17-year-olds, according to CDC data ending April 30.