The Food and Drug Administration is preparing to authorize use of the Pfizer-BioNTech Covid-19 vaccine in adolescents 12 to 15 years old by early next week, according to federal officials familiar with the agency’s plans, opening up the nation’s vaccination campaign to millions more Americans. The news is highly anticipated: Eager parents have been counting down the weeks since Pfizer announced results from its trial in adolescents, showing the vaccine is at least as effective in that age group as it is in adults. Vaccinating children is also key to raising the level of herd immunity in the population and bringing down the numbers of hospitalizations and deaths. The authorization could come as early as late this week, according to the federal officials, who did not give their names because they weren’t authorized to speak publicly.