Background Influenza and respiratory syncytial virus (RSV) are major contributors to the burden of seasonal influenza-like illnesses (ILI) in the US. The prevention and treatment of ILI varies substantially across age groups and in cost and administration schedule. Clearly identifying the times when healthcare resources are most needed to mitigate the effects of seasonal RSV and influenza outbreaks will improve public health responses before and during ILI seasons. Methods We implemented stacked-regression linear models to infer the contribution of each of these diseases to seasonal ILI syndromic indicators. We further implemented anomaly-detection algorithms on data from the US Centers for Disease Control and Prevention National Syndromic Surveillance Program to identify the timing of onsets and peaks of RSV, influenza, and COVID-19. Findings A total of 148 state-ILI seasons were analyzed. In 114 out of 148 (77.0%) of analyzed seasons, volume of RSV emergency department (ED) visits peaked before influenza ED visits. The median time difference between peaks of RSV and peaks of influenza was +3.0 weeks. The timing of RSV and influenza onsets were found to occur more synchronously in the 2023-2024 and 2024-2025 ILI seasons. Interpretations RSV epidemics frequently reach peak volume before influenza epidemics across the US. Healthcare professionals and public health authorities should anticipate increases in RSV cases and hospitalizations at the start of the annual ILI season and establish infrastructure and planning to handle incoming surges of both RSV and influenza appropriately. Funding No specific funding was provided for this study. Summary Epidemics of respiratory pathogens such as influenza or RSV drive the influenza-like illness season in the US. We show that RSV epidemics peak before influenza epidemics in most states, with about a one to three week difference separating the epidemics.
