This retrospective study uses a hybrid design with three analytic components: 1) characterization of the entire cohort with regard to demographics, eligible time under observation, immunization status, incidence rates of GBS during various categories of person time (never immunized, before immunization, during the 42 days after immunization, etc.), and comorbidities and other covariates, 2) a nested case-control study assessing risk associated with immunization, and 3) a self-controlled case series assessing immunization associated risk.