All potential cases of GBS among the cohort members will be identified, and a sample of 100 non-cases from the same risk set as each case will be randomly selected and matched on age, sex, and state of residence. Their vaccination histories and other covariates will be included in analytic datasets to be transferred to the coordinating center for pooled multivariate analysis. Covariates will be estimated as of an index date assigned within each case-control set which is the date of GBS onset for the associated case.