At least 600,000 children, ages 12 to 15, have received their first dose of the Pfizer-BioNTech Covid-19 vaccination, Dr. Rochelle Walensky, director of the CDC said during a media briefing Tuesday. The count comes just over a week after the FDA greenlighted it for emergency use in this age group, a total of 17 million in the U.S. — though many major pharmacies and hospitals did not start administering the shots until last Thursday, after the CDC also signed off on it.