An additional case-series analysis (to be referred to as "Analysis C") will be performed among the subset of adjudicated GBS cases with prior MCV4 vaccination. Date of MCV4 vaccination will serve as the index date for each patient. The relative incidence rate of GBS during a "control" period of 43 or more days following vaccination will be compared to the incidence during the "exposed" risk window of 1-42 days following vaccination using log linear modeling with terms for the exposed and control periods.