The U.S. is expanding COVID-19 boosters, opening a third dose of Pfizer’s vaccine to people as young as age 16. The U.S. and many other nations already urge adults to get a booster to pump up their immunity months after initial vaccination. In the U.S., the Pfizer vaccine is the only option for children and teenagers. The Food and Drug Administration on Thursday cleared an extra dose for those ages 16 and 17, and the Centers for Disease Control and Prevention rapidly endorsed a booster for those ages. It’s not clear if younger teens will need one.