U.S. health authorities again expanded the nation’s booster campaign Thursday, opening extra doses of Pfizer’s COVID-19 vaccine to several million 16- and 17-year-olds. … On Thursday, the FDA gave emergency authorization for 16- and 17-year-olds to get a third dose of the vaccine made by Pfizer and its partner BioNTech — once they’re six months past their last dose. And hours later, the CDC lifted the last barrier …