The leading cause of infant hospitalizations during in-season (November–March) and out-of-season (April–October) months by time period, with percentage of total infant hospitalizations and corresponding 95% confidence intervals, National (Nationwide) Inpatient Sample, 2009–2019. In-season refers to respiratory syncytial virus (RSV) seasonality. Percentage of total infant hospitalizations for acute bronchiolitis due to RSV was provided for the out-of-season months as the ninth-ranked and fourth-ranked primary diagnosis in the 2 time periods, respectively.