Federal regulators authorized a coronavirus booster shot Tuesday for school-aged children, making a third shot of the Pfizer-BioNTech vaccine available to 5-to-11-year-olds as cases rise nationally.

The Food and Drug Administration cleared the booster for use at least five months after children are fully vaccinated with the two-shot primary series.

FDA Commissioner Robert M. Califf said in a statement that while COVID-19 tends to be less severe in children, the wave of infections caused by the omicron variant has resulted “in more children getting sick with the disease and being hospitalized, and children may also experience longer term effects, even following initially mild disease.” He said the agency was authorizing the booster to provide continued protection.

Advisers to the Centers for Disease Control and Prevention are scheduled to meet Thursday and are expected to recommend the booster, which was shown in laboratory tests to strengthen children’s immune defenses – particularly against the omicron variant.

The shot from Pfizer and its German partner, BioNTech, is the only vaccine available to children in this age group, and two shots have provided disappointing protection to children in this age group in real-world studies. Protection against both infections and hospitalizations waned quickly in children.

The FDA said its decision was based on laboratory measurements of the immune responses in 67 children. A month after the booster dose, their levels of virus-blocking antibodies were higher than they were before the shot, the agency said. Pfizer’s trial includes 4,500 children in this age group.

A third shot could help shore up children’s immune defenses, but the impact of a third shot will probably be limited. Only 28% of children in this age group are fully vaccinated, according to CDC. More than 18 million children in the group have not received a single shot yet, despite shots being available to them since November.

In January, the FDA authorized a booster of the vaccine for adolescents 12 through 15 years old.

Tuesday’s move may sharpen some parents’ frustration about the divide between children who are able to get a third shot and those who can’t yet get a dose. Children younger than 5 still lack access to any vaccine. A top FDA official predicted in an interview that the first shots might become available in June, assuming data supports such a move.

In late April, vaccine maker Moderna asked the FDA for emergency authorization for its two-shot coronavirus vaccine for children 6 months to 5 years old. Pfizer and BioNTech are expected to submit the full data on a three-dose regimen for children 6 months to 4 years old in coming weeks.

The FDA has set aside several dates – June 8, 21 and 22 – to meet with its outside vaccine advisers – the Vaccines and Related Biological Products Advisory Committee – to discuss applications for pediatric vaccines.

The FDA also announced plans to meet June 7 with its advisers to talk about 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.

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 for use in a potential coronavirus vaccination campaign this fall.

Many experts say vaccines 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.