We are conducting a large, record-based study to assess the risk of Guillain-Barré Syndrome (GBS), a serious demyelinating disease, following immunization with the tetravalent meningococcal conjugate vaccine (Menactra) that is currently recommended for all adolescents. The study was requested by CDC and FDA because of an unexpected number of case reports to the CDC's Vaccine Adverse Event Reporting System (VAERS).