Uncovering the post-pandemic timing of influenza, RSV, and COVID-19 driving seasonal influenza-like illness in the United States: a retrospective ecological study

TL;DR

This study examines the timing of RSV, influenza, and COVID-19 outbreaks in the US to better prepare for seasonal respiratory illnesses.

Contribution

The study identifies RSV often peaking before influenza, offering insights for public health planning.

Findings

RSV emergency department visits peaked before influenza in 77% of analyzed seasons.

The median time difference between RSV and influenza peaks was 3 weeks.

RSV and influenza onsets became more synchronized in recent seasons.

Abstract

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. This study aimed to characterize the timing and ordering of RSV, influenza, and COVID-19 epidemics in the post-pandemic period to inform public health preparedness.

We implemented a series of independent regression 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 for the 2022–23, 2023–24, and 2024–25 ILI seasons to identify the timing of onsets and peaks of RSV, influenza, and COVID-19.

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 (95% percentile range: −7.0, +7.0 weeks; interquartile range: 5.0 weeks). The timing of RSV and influenza onsets were found to occur more synchronously in the 2023–2024 and 2024–2025 ILI seasons. The timing of COVID-19 outbreaks did not show a consistent seasonal pattern across the study period.

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.

CDC 10.13039/100025287Center for Forecasting and Outbreak Analytics; 10.13039/100000002National Institutes of Health.

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