Adverse events were collected through the Adverse Events Following Immunization (AEFI) Surveillance System. Beijing CDC and local prefectural CDC must organize an expert panel to investigate adverse events and assess causality, using criteria based on Chinese Standard Procedures for Vaccination. The panels consist of physicians, epidemiologists, pharmacists, and other relevant experts. In general, expert panels investigate deaths, life-threatening illnesses, and permanent disabilities, and immunization-program managers or vaccination providers investigate common, minor adverse events. Adverse events are classified into one of five categories: vaccine reactions (common and minor to rare and more serious), program errors, coincidental illnesses, psychogenic reactions, and unclassifiable events.