Emergence of Japanese encephalitis among adults 40 years of age or older in northern China: Epidemiological and clinical characteristics.

Japanese encephalitis (JE) is a vector-borne vaccine preventable infectious disease for which vaccine provides direct protection. China introduced nationwide JE vaccination of young children in 2007, rapidly achieving high coverage. In 2018, 1,800 JE cases were reported in China, with morbidity and mortality rates of 0.13/100,000 and 0.0097/100,000. Nationally, 64% of cases were among adults aged 40 years or older, and in northern China, 82% were among adults, mainly in outbreaks. Severity varied little by age grouping. Compared with the previous 10 years, the proportion and absolute number of cases among adults increased, indicating possible emergence of adult JE in China as childhood JE decreased. We describe JE epidemiology and clinical features nationally and in areas with low endemicity after more than 10 years of routine childhood JE vaccination. Determining population JE seroprevalence should be a priority; vaccination campaigns among adults in high risk areas should be considered.