Moderna is seeking to be the first to offer COVID-19 vaccine for the youngest American children, as it asked the Food and Drug Administration Thursday to clear low-dose shots for babies, toddlers and preschoolers.

Frustrated families are waiting impatiently for a chance to protect the nation’s littlest kids as all around them people shed masks and other public health precautions — even though highly contagious coronavirus mutants continue to spread. Already about three-quarters of children of all ages show signs they’ve been infected at some point during the pandemic.

Moderna submitted data to the Food and Drug Administration that it hopes will prove two low-dose shots can protect children younger than 6 — although the effectiveness wasn’t nearly as high in kids tested during the omicron surge as earlier in the pandemic.

“There is an important unmet medical need here with these youngest kids,” Dr. Paul Burton, Moderna’s chief medical officer, told The Associated Press. Two kid-size shots “will safely protect them. I think it is likely that over time they will need additional doses. But we’re working on that.”

Moderna said two kid doses were about 40% to 50% effective at preventing symptomatic COVID-19, not a home run but for many parents, any protection would be better than none.

That effectiveness is “less than optimal. We were hoping for better efficacy but this is a first step,” said Dr. Nimmi Rajagopal of Cook County Health in Chicago. She’s anxiously awaiting vaccinations for her youngest patients and her own 3-year-old son who’s ready to enter preschool.

“It gives me such peace of mind to know that hopefully by fall I’ll get him in school and he’ll be fully vaccinated,” she said.

Now, only children ages 5 or older can be vaccinated in the U.S., using rival Pfizer’s vaccine, leaving 18 million younger tots unprotected.

Moderna’s vaccine isn’t the only one in the race. Pfizer is soon expected to announce if three of its even smaller-dose shots work for the littlest kids, months after the disappointing discovery that two doses weren’t quite strong enough.

Whether it’s one company’s shots or both, FDA vaccine chief Dr. Peter Marks said the agency will “move quickly without sacrificing our standards” in deciding if tot-sized doses are safe and effective.

While questions are swirling about what’s taking so long, Marks pointedly told lawmakers earlier this week that the FDA can’t evaluate a product until a manufacturer completes its application.