Japanese encephalitis (JE) virus is estimated to result in 3500-50,000 clinical cases every year, with mortality rates of up to 20-50% and a high percentage of neurological sequelae in survivors. Vaccination is the single most important measure in preventing this disease. Inactivated Vero cell culture-derived JE vaccines have not been linked to any fatalities, and few serious adverse events after vaccination have been reported. Here, we report a case of sudden death in which a 10-year-old boy experienced cardiopulmonary arrest 5 min after receiving a Japanese encephalitis vaccination. He had been receiving psychotropic drugs for the treatment of pervasive developmental disorders. Postmortem examinations were nonspecific, and no signs of dermatologic or mucosal lesions or an elevation of the serum tryptase level, which are characteristic of anaphylaxis, were observed. A toxicological examination revealed that the blood concentrations of the orally administered psychotropic drugs were within the therapeutic ranges. The patient was considered to have died of an arrhythmia that was not directly associated with the vaccination.

Kikuchi-Fujimoto disease (KFD) is a benign and self-limited syndrome, characterized by regional lymphadenopathy and fever. Although the pathogenesis of KFD remains unclear, infectious agents, autoimmune causes, and physicochemical factors have been suggested as triggers. However, KFD following vaccination has never been reported. We present a 14-year-old girl who suffered from fever and cervical lymph node swelling following simultaneous administration of human papilloma virus vaccine and Japanese encephalitis virus vaccine. The patient was diagnosed with KFD based on the histopathologic findings of a lymph node biopsy, and her fever and swelling resolved with oral corticosteroid therapy. Although the exact pathogenesis of the development of KFD following immunization remains unknown, this should be added to the list of potential triggers or factors associated with the development of KFD.