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. Respiratory syncytial virus (RSV) infection is not included but its incorporation is needed for effective early epidemic detection. We aimed to propose criterion values for an epidemic warning system related to 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 to establish an index for epidemic warnings. We determined criteria for issuing an epidemic warning by identifying distribution percentiles within 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, which aligns with previously accepted frequency criteria for epidemic warnings. The 5-year (2015-2019) RSV infection epidemic trend showed that the set critical values were appropriate. Using these values could assist with issuing warnings regarding potential RSV infection epidemics in Japan.