U.S. regulators on Monday expanded the use of Pfizer’s COVID-19 vaccine to children as young as 12, offering a way to protect the nation’s adolescents before they head back to school in the fall and paving the way for them to return to more normal activities. Shots could begin as soon as a federal vaccine advisory committee issues recommendations for using the two-dose vaccine in 12- to 15-year-olds. An announcement is expected Wednesday. … Pfizer’s vaccine is being used in multiple countries for teens as young as 16, and Canada recently became the first to expand use to 12 and up.