U.S. regulators on Thursday cleared doses of the updated COVID-19 vaccines for children younger than age 5.

The Food and Drug Administration's decision aims to better protect the littlest kids amid an uptick in COVID-19 cases around the country — at a time when children's hospitals already are packed with tots suffering from other respiratory illnesses including the flu.

"Vaccination is the best way we know to help prevent the serious outcomes of COVID-19, such as hospitalization and death," Dr. Peter Marks, FDA's vaccine chief, told The Associated Press.

Omicron-targeted booster shots made by Moderna and rival Pfizer already were open to everyone 5 and older.

The FDA now has authorized use of the tweaked shots starting at age 6 months — but just who is eligible depends on how many vaccinations they've already had, and which kind. Only about 5% of youngsters under age 5 have gotten the full primary series since vaccinations for the littlest kids began in June.

The FDA decided that:

--Children under age 6 who've already gotten two original doses of Moderna's COVID-19 vaccine can get a single booster of Moderna's updated formula if it's been at least two months since their last shot.

--Pfizer's vaccine requires three initial doses for tots under age 5 — and those who haven't finished that vaccination series will get the original formula for the first two shots and the omicron-targeted version for their third shot.

The Centers for Disease Control and Prevention is expected to sign off soon, the final step for shots to begin.

Marks said the bivalent vaccine is safe for tots and will help parents "keep the protection for those children as up to date as possible."

But children under 5 who already got all three Pfizer doses aren't yet eligible for an updated booster.

For now, "the good news is they are probably reasonably well-protected," Marks said.

The FDA expects data from Pfizer and its partner BioNTech sometime next month to determine whether those tots will need an omicron-targeted booster "and we will act on that as soon as we can," he said.

For parents who haven't yet gotten their children vaccinated, it's not too late — especially as "we are entering a phase when COVID-19 cases are increasing," Marks said.

The updated vaccines from Moderna and Pfizer are combination shots, containing half the original vaccine and half tweaked to match the BA.4 and BA.5 omicron strains that until recently were dominant. Now BA.5 descendants are responsible for most COVID-19 cases.

The CDC last month released the first real-world data showing that an updated booster, using either company's version, does offer added protection to adults. The analysis found the greatest benefit was in people who'd never had a prior booster, just two doses of the original COVID-19 vaccine — but that even those who'd had a summertime dose were more protected than if they'd skipped the newest shot.