Pfizer Inc. has asked U.S. regulators for emergency-use authorization of a booster shot of its COVID-19 vaccine in children ages 5 to 11, setting in motion an effort to provide extra protection to kids.

       Pfizer and partner BioNTech submitted data to the Food and Drug Administration from a late-stage study that showed a third-dose booster shot, given about 6 months after the second dose, provided a strong immune response.

       The companies also plan to submit data to the European Medicines Agency and other regulators around the world, according to a statement Tuesday. No new safety concerns were identified, they said.

       The U.S. campaign to immunize children has tapered off, with 28% of children ages 5 to 11 fully vaccinated, according to the Centers for Disease Control and Prevention. Fewer than a quarter of adolescents 12 to 17, who are already eligible for boosters, have received one. That bodes poorly for a booster campaign among the youngest children, experts say