Pfizer is asking U.S. regulators to authorize its updated COVID-19 vaccine for children under age 5 - not as a booster but part of their initial shots.

Children ages 6 months through 4 years already are supposed to get three extra-small doses of the original Pfizer COVID-19 vaccine - each a tenth of the amount adults receive - as their primary series. If the Food and Drug Administration agrees, a dose of Pfizer's bivalent omicron-targeting vaccine would be substituted for their third shot.

Pfizer and its partner BioNTech said Monday that may help prevent severe illness and hospitalization from COVID-19 in little kids, at a time when children’s hospitals already are packed with youngsters hit by other respiratory illnesses.

Few of the nation's youngest children have gotten their COVID-19 vaccinations since the shots were OK'd in June: Just 2% of tots under 2 and about 4% of 2- to 4-year-olds have gotten their primary doses so far, according to the Centers for Disease Control and Prevention.

The FDA has authorized the new bivalent COVID-19 shots - versions made by Pfizer and rival Moderna - as a booster for everyone ages 5 and older. Those combination shots contain 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.

By LAURAN NEERGAARD