Pfizer said its COVID-19 vaccine was 100% effective in a final-stage trial in kids ages 12 to 15, a finding that could pave the way for shots for teens and pre-teens before the next school year.The vaccine is already authorized in the U.S. for people ages 16 and up. Pfizer and its partner BioNTech said they planned to submit the data to regulators in the U.S.