Spatial and temporal variability of respiratory syncytial virus disease seasonality in Japan, 2012–2024

BACKGROUND: Understanding infectious disease seasonality is crucial to inform immunization timing and risk communications. This study aimed to describe trends in respiratory syncytial virus (RSV) seasonality in Japan by determining thresholds across 47 prefectures using public health surveillance data. METHODS: Weekly RSV cases per sentinel (CPS) is the main indicator used in Japan. Data from approximately 3000 pediatric sentinel sites were extracted for 2012-2024. Seasonal, advisory, and alert thresholds were established via two globally utilized methods (moving epidemic methods [MEM] and fixed threshold method). Based on the established seasonal threshold, onset/duration was examined for each season during the study period. RESULTS: The fixed threshold method was finally chosen to establish the seasonal threshold, while medium and high-intensity thresholds under the MEM methods were chosen as advisory and alert thresholds. The threshold values were 0.23-0.80CPS for seasonal, 0.75-2.82CPS for advisory, and 1.11-4.20CPS for alert. The epidemic periods usually lasted less than 6 months, with distinct epidemic peaks in almost all prefectures over the 13-year observation. However, unlike other temperate countries/regions such as the United States and Europe, season onset shifted drastically from September-October in 2012/2013 to around March-April in 2024 with geographic variabilities. Calling season onset after 2 consecutive weeks above the threshold resulted in no false alarms in over half the prefectures. CONCLUSIONS: In countries such as Japan and the tropics/subtropics where the RSV seasons are less predictable, a flexible prevention strategy, tailored for each region/prefecture, using thresholds as guides, would ensure optimal protection against RSV and maximize public health benefits regardless of seasonal variability.