In many parts of the country, if you want your child to be immunized against Covid-19 by the time classes start, you need to act fast. … It takes five weeks to be fully vaccinated with Pfizer's vaccine, the only one authorized for adolescents ages 12 to 17. That means, for example, Atlanta students need to get their first shot by July 1 to be fully immunized by the first day of school on August 5.