U.S. regulators on Monday expanded the use of Pfizer’s COVID-19 vaccine to children as young as 12. 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. Most COVID-19 vaccines worldwide have been authorized for adults. 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. The Food and Drug Administration declared that the Pfizer vaccine is safe and offers strong protection for younger teens based on testing of more than 2,000 U.S. volunteers ages 12 to 15. The study found no cases of COVID-19 among fully vaccinated adolescents compared to 18 among children given dummy shots. Researchers also found the children developed higher levels of virus-fighting antibodies than earlier studies measured in young adults. The …