A group of medical experts said children ages 12 to 15 years old can safely take the Covid-19 vaccine made by Pfizer Inc. and BioNTech SE, opening an important new phase of the U.S. immunization effort. New York-based Pfizer and Germany’s BioNTech said in March that their two-shot vaccine was 100% effective in the 12-to-15 age group in a final-stage trial. The shot produced antibodies exceeding the level seen in vaccinated young adults, and didn’t result in any new or worrisome side effects. Pfizer shares closed in the green on Wednesday with gains since the beginning of the year at 7,82%.