The Food and Drug Administration on Monday authorized use of the Pfizer-BioNTech Covid-19 vaccine for 12- to 15-year-olds in the United States, a crucial step in the nation’s steady recovery from the pandemic and a boon to tens of millions of American families eager for a return to normalcy. The authorization caps weeks of anticipation among parents, who have been grappling with how to conduct their lives when only the adults in a household are immunized. It removes an obstacle to school reopenings by reducing the threat of transmission in classrooms, and affords millions of adolescents the opportunity to attend summer camps, sleepovers and get-togethers with friends. “This is great news,” said Dr. Kristin Oliver, a pediatrician and vaccine expert at Mount Sinai Hospital in New York. “It feels like we’ve been waiting a long time to start protecting children in this age group.” The Pfizer-BioNTech vaccine is already available to anyone over 16.