Pfizer and BioNTech said Thursday they are beginning a study aimed at showing their Covid-19 vaccine can be used in children as young as 6 months. The study follows the launch of a separate and ongoing trial in children ages 12 to 15, which was fully enrolled in January. That study could lead to results by the end of the first half of the year, depending on the data, and then to an emergency use authorization. That will depend on the FDA and the CDC.