Pfizer asked U.S. regulators Monday to expand use of its updated COVID-19 booster shot to children ages 5 to 11. Elementary school-aged children already received kid-sized doses of Pfizer’s original vaccine, a third of the dose given to everyone 12 and older — two primary shots plus a booster. If the Food and Drug Administration agrees, they would start getting a kid-sized dose of the new omicron-targeted formula when it’s time for their booster. FDA vaccine chief Dr. Peter Marks said last week he expected a decision soon on boosters for that age group. Pfizer and its partner BioNTech also ann…