LOS ANGELES, CA – AUGUST 07: A school nurse preapres a vaccine against whooping cough before giving it to students at Mark Twain Middle School August 7, 2012 in Los Angeles, California. The boosters, also called Tdap shots, are required of all seventh graders before they can start school. The Los Angeles Unified School District is offering free shots at various clinics in the city to help students make the deadline before school start on August 14. (Photo by Kevork Djansezian/Getty Images)
