Abstract:Abstract: OBJECTIVE To determine whether thrombocytopenia observed in a pediatric patient following administration of JE (Japanese Encephalitis)liveattenuated vaccine and MMR(Measles,Mumps,Rubella)live attenuated vaccine was vaccine-related and to assess for suspected adverse events following immunization(AEFI).METHODS An individual case investigation was conducted. Data collected included the patient's birth records,admission medical records,vaccine manufacturer's certificate of conformity, batch-specific vaccine quality inspection reports,and vaccination station operating procedures.These materials were submitted to the municipal ADR expert investigation and diagnosis team(comprising specialists in hematology,epidemiology,pediatrics,etc.).A comprehensive analysis was performed based on the Principles for Determining Adverse Reactions to Immunization to reach a conclusion. RESULTS The child,who had received JE and MMR vaccines,completed two doses on February 14,2025,and underwent a 30-minute observation period without adverse reactions.The child was admitted for treatment starting the following day until February 17 due to thrombocytopenia,with the discharge diagnosis being “thrombocytopenia and acute phase of respiratory disease”.The investigation confirmed that the vaccine batch met quality standards and that all vaccination procedures strictly adhered to the Standard Operating Procedures for Immunization.The municipal expert panel concluded that this case did not constitute a vaccine-related adverse reaction.CONCLUSION The child's thrombocytopenia was not causally linked to vaccination and represented a coincidental reaction. This highlights the clinical need for differential diagnosis of coincidental illnesses following vaccination.