If a child develops encephalitis during the weeks to months following CD-JEV vaccination, and JE IgM antibodies are detected in serum in the absence of a diagnostic cerebrospinal fluid (CSF) sample, the question arises whether the serum IgM suggests a wild-type JE virus infection indicating vaccine failure or reflects postvaccination IgM persistence. The gene discussed is CD40LG; the disease is viral encephalitis.