Seasonality and Trend of Cellulitis, Herpes Zoster, and Varicella: A Nationwide Population-based Study.

Introduction: Cellulitis, herpes zoster, and varicella are common infectious skin diseases with significant public health implications. While these conditions have been studied in some countries, comprehensive nationwide data on their seasonality and trends in Japan remain limited. Understanding these patterns is essential for effective public health planning and disease management. Methods: This study analyzed monthly outpatient data for cellulitis, herpes zoster, and varicella using aggregated summary tables from the National Database of Health Insurance Claims of Japan for fiscal years 2014 to 2019. Seasonal patterns and trends were examined using seasonal decomposition of time series by locally estimated scatterplot smoothing. Results: Distinct seasonal patterns were observed for the three diseases. Both cellulitis and herpes zoster peaked during the summer months, while varicella exhibited a winter predominance. Trend analysis revealed a 13.5% increase in cellulitis cases over the study period. Herpes zoster cases also increased, whereas varicella cases showed a significant decline between fiscal years 2014 and 2015, coinciding with the introduction of universal varicella vaccination. The seasonal patterns were consistent with findings from previous studies in other countries. Conclusions: This nationwide study confirmed distinct seasonal patterns and trends for cellulitis, herpes zoster, and varicella in Japan. The findings enhance the understanding of the epidemiology of these infectious skin diseases at a nationwide population level and provide valuable insights for public health planning.