Utilizing infectious disease surveillance for epidemic warnings of respiratory syncytial virus infections in Japan from 2015 to 2019

Japan's National Epidemiological Surveillance of Infectious Diseases (NESID) has implemented a warning system for detecting epidemics in smaller districts, such as public health center (PHC) areas. This system is applied to influenza and pediatric infectious diseases but not to respiratory syncytial virus (RSV) infection, whose incorporation is essential for early epidemic detection. We aimed to propose criterion values for the epidemic warning system of RSV infection within PHC areas. We analyzed the weekly number of RSV infection cases reported by sentinel medical institutions (SMIs) in the NESID from 2015 to 2019. Weekly cases per SMI in the PHC area were set for an index for epidemic warnings. We determined the criteria for issuing an epidemic warning by describing the percentiles of the distribution of the index. Setting the critical values for the onset and end of the epidemic warning at 5 and 2, respectively, yielded an approximately 5% annual proportion of PHC areas with an epidemic warning. This aligns with the acceptably implemented frequency of epidemic warnings. The 5-year (2015-2019) RSV infection epidemic trend showed that the set critical values were appropriate. Using these values could help issue warnings regarding potential RSV infection epidemics in Japan.