Adverse events were collected through the National Immunization Information System's National Adverse Events Following Immunization (AEFI) Surveillance System, which was established in 2005 on the basis of World Health Organization (WHO) guidelines. According to the Guideline for the Identification of Adverse Reaction after Immunization issued by the Chinese Ministry of Health in 2008, Sichuan 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.