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Epidemiology of RSV in Japan: A nationwide claims database analysis. BACKGROUND: Respiratory syncytial virus (RSV) is a major cause of hospitalization for bronchiolitis and pneumonia in infancy. In Japan, limited data are publicly available on RSV epidemiology and clinical characteristics among infants. METHODS: This retrospective study described RSV incidence, seasonality, patient characteristics, resource use, and clinical outcomes among Japanese children <2 years from January 2017 through December 2018. RSV cases were identified using the Japanese Medical Data Center database. RESULTS: In the database, 9,711 and 8,509 RSV patients <2 years were identified in 2017 and 2018. Of these, 25% required hospitalization. Ninety percent of hospitalized patients did not have a known RSV risk factor. Nineteen percent of hospitalized patients experienced dehydration, and 12% had acute respiratory failure. Hospitalization lasted one week on average and 7% required some type of mechanical ventilation. The peak of hospitalizations occurred at 2 months. The incidence of RSV hospitalization in children <2 years was 23.2 per 1000 person-years, which increased to 35.4 per 1000 for infants <6 months. This age group accounted for 40% of all RSV-associated hospitalizations among children <2 years. CONCLUSIONS: Roughly one-fourth of all RSV patients <2 years were hospitalized. Ninety percent of these did not have an underlying risk condition. This underscores that RSV can cause serious disease among all young children. Three to four out of every 100 Japanese children <6 months were hospitalized for RSV, and this age group accounted for ~40% of all RSV-associated hospitalizations. Novel and broad-based RSV prevention strategies, especially those targeting young infants, are needed.