A health worker administers a dose of a Moderna COVID-19 vaccine during a vaccination clinic at the Norristown Public Health Center in Norristown, Pa., Tuesday, Dec. 7, 2021. Moderna says its COVID-19 vaccine works in babies, toddlers and preschoolers. The company announced early findings from a study of children younger than 6 on Wednesday, March 23, 2022. Photo: AP/Matt Rourke

Moderna said in the coming weeks it would ask regulators in the U.S. and Europe to authorize two small-dose shots for youngsters under 6. The company also is seeking to have larger-dose shots cleared for older children and teens in the U.S.

The nation’s 18 million children under 5 are the only age group not yet eligible for vaccination. Competitor Pfizer currently offers kid-sized doses for school-age children and full-strength shots for those 12 and older.

But parents have anxiously awaited protection for younger tots, disappointed by setbacks and confusion over which shots might work and when. Pfizer is testing even smaller doses for children under 5 but had to add a third shot to its study when two didn’t prove strong enough. Those results are expected by early April.

Vaccinating the littlest “has been somewhat of a moving target over the last couple of months,” Dr. Bill Muller of Northwestern University, an investigator in Moderna’s pediatric studies, said in an interview before the company released its findings. “There’s still, I think, a lingering urgency to try to get that done as soon as possible.”

The younger the child, the smaller the dose being tested. Moderna said a quarter of the dose it uses for adults worked well for youngsters under age 6.

Moderna enrolled about 6,900 tots in a study of the 25-microgram doses. Early data showed after two shots, youngsters developed virus-fighting antibody levels just as strong as young adults getting regular-strength shots, the company said in a press release.

Moderna said the small doses were safe, and the main side effects were mild fevers like those associated with other commonly used pediatric vaccines.