Sites will use programs distributed by the coordinating center to compute incidence rates of GBS according to age, sex, season, and MCV4 vaccination status among the full base population (health plan enrollees who meet age and cohort eligibility criteria). Additionally, vaccination rates for MCV4 according to age, sex, and calendar year will be computed among all cohort members. Incidence rates of GBS provided by these analyses will be used to calculate the attributable risk of GBS following MCV4 vaccination should a significant relationship be identified.