Highlights
- RSV emergency department visits peaked before influenza in 77.0% of analyzed state-seasons.
- The median time difference between RSV and influenza peaks was 3.0 weeks, highlighting a consistent temporal lead for RSV.
- Viral onsets showed increased synchronicity during the 2023-2024 and 2024-2025 seasons compared to earlier post-pandemic years.
- COVID-19 outbreaks continue to exhibit non-seasonal patterns, complicating the traditional influenza-like illness (ILI) predictability.
Introduction: The Post-Pandemic Respiratory Landscape
For decades, the cadence of seasonal respiratory viral infections in the United States followed a relatively predictable pattern. However, the emergence of SARS-CoV-2 and the subsequent public health interventions fundamentally disrupted the ecological niches and transmission dynamics of endemic pathogens, particularly influenza and respiratory syncytial virus (RSV). As the United States transitions into a post-pandemic “new normal,” clinicians and public health authorities face the challenge of managing a “triple-threat” of viruses that drive influenza-like illness (ILI) morbidity and mortality.
Understanding the precise timing and ordering of these epidemics is not merely an academic exercise; it is a clinical necessity. Effective healthcare delivery depends on the strategic timing of vaccination campaigns, the allocation of pediatric and adult intensive care beds, and the procurement of antiviral therapeutics. This retrospective ecological study provides a high-resolution analysis of how RSV, influenza, and COVID-19 interact temporally across the modern U.S. landscape.
Study Design and Methodology
The researchers utilized data from the U.S. Centers for Disease Control and Prevention (CDC) National Syndromic Surveillance Program (NSSP), focusing on emergency department (ED) visits for ILI. The study period spanned three distinct respiratory seasons: 2022-23, 2023-24, and 2024-25, covering a total of 148 state-level ILI seasons.
To differentiate the specific contributions of each virus to the broader ILI syndromic indicators, the team implemented a series of independent regression models. Furthermore, anomaly-detection algorithms were employed to pinpoint the exact onset and peak of each viral surge. This dual-methodological approach allowed for a robust characterization of viral behavior across different geographic and demographic contexts within the United States.
Key Findings: The Sequential Nature of ILI
RSV: The Seasonal Harbinger
The most striking finding of the study was the temporal precedence of RSV. In 114 out of the 148 analyzed state-seasons (77.0%), RSV ED visit volume reached its peak before influenza. This suggests that in the majority of the U.S., the initial strain on the healthcare system during the ILI season is driven by RSV rather than influenza. The median time difference between the peak of RSV and the peak of influenza was +3.0 weeks. While the interquartile range was 5.0 weeks, the 95th percentile range spanned from -7.0 to +7.0 weeks, indicating that while RSV usually leads, there are instances of near-simultaneous peaks or influenza-led seasons.
Increasing Synchronicity in Recent Seasons
While the early post-pandemic period was marked by erratic viral timing, the data from the 2023-2024 and 2024-2025 seasons suggest a return to more synchronous viral onsets. This synchronization means that the window between the first rise in RSV cases and the first rise in influenza cases is narrowing. For healthcare systems, this implies a shorter preparation window and a higher likelihood of overlapping surges, which can strain laboratory diagnostics and clinical staffing.
The Non-Seasonal Nature of COVID-19
Unlike RSV and influenza, which showed clear, albeit shifting, seasonal signatures, COVID-19 did not exhibit a consistent seasonal pattern across the study period. Outbreaks occurred at various times throughout the year, independent of the traditional winter ILI peak. This lack of seasonality suggests that COVID-19 remains a “wildcard” in public health planning, requiring year-round vigilance rather than the focused seasonal mobilization used for influenza.
Clinical and Public Health Implications
The finding that RSV typically peaks three weeks prior to influenza has profound implications for clinical practice. This three-week window is a critical period for hospital systems to adjust their surge capacity. Because RSV disproportionately affects the very young and the elderly, pediatric hospitals in particular must be prepared for peak volumes earlier than the traditional “flu season” might suggest.
Furthermore, these findings inform the timing of preventive measures. With the recent approval of RSV vaccines for older adults and monoclonal antibody treatments for infants, the timing of administration is paramount. If RSV consistently peaks in late autumn or early winter, vaccination campaigns must be initiated early enough to ensure peak immunity before the RSV surge, which now appears to be the primary driver of early-season ILI.
Expert Commentary
Medical experts note that the ecological competition between these viruses is still evolving. The “interference” phenomenon—where an infection with one virus may temporarily protect a population from another through innate immune activation—might contribute to the sequential peaks observed. However, the study’s data on increasing synchronicity suggests that this interference may be weakening as population immunity to all three viruses stabilizes. The study highlights the necessity of multiplex molecular testing in ED settings to rapidly distinguish between these pathogens, as clinical symptoms of RSV, influenza, and COVID-19 are often indistinguishable but require different management protocols.
Conclusion
This retrospective study clarifies the post-pandemic order of respiratory viral circulation in the U.S., establishing RSV as the frequent precursor to influenza peaks. While the two viruses are increasingly occurring in tandem, the consistent three-week lead time for RSV offers a actionable window for healthcare planning. Meanwhile, the unpredictable nature of COVID-19 necessitates a flexible, multi-pathogen approach to respiratory disease surveillance. Public health infrastructure must be designed to handle the “long tail” of COVID-19 alongside the concentrated, sequential surges of RSV and influenza.
References
- Dewey G, Meyer AG, Garcia RG, Santillana M. Uncovering the post-pandemic timing of influenza, RSV, and COVID-19 driving seasonal influenza-like illness in the United States: a retrospective ecological study. Lancet Reg Health Am. 2026 Jan 1;55:101359. doi: 10.1016/j.lana.2025.101359.
- Centers for Disease Control and Prevention. National Syndromic Surveillance Program (NSSP). https://www.cdc.gov/nssp/index.html
- Prasad N, et al. Respiratory Syncytial Virus-Associated Hospitalizations Among Adults: 2018–2023. JAMA Network Open. 2024.
Funding
This research was supported by the CDC Center for Forecasting and Outbreak Analytics and the National Institutes of Health (NIH).
