U.S. RSV Season Tracking Holds Up Under Postpandemic Pressure
Respiratory syncytial virus seasonality is not a bookkeeping exercise. It shapes when infants, older adults, and other high-risk patients can be protected with prevention products, and a late read on local circulation can leave vulnerable patients exposed during the first weeks of a rising epidemic. In a decade-long U.S. surveillance analysis, the familiar 3% laboratory percent positivity threshold remained a workable tool for defining RSV season onset after the pandemic period, even when compared with two retrospective analytic approaches that often identified circulation earlier.
The study examined laboratory testing data submitted by consistent reporters to the National Respiratory and Enteric Virus Surveillance System from July 2014 through June 2024. Investigators compared three ways of defining an epidemic period: the 3% threshold, the retrospective slope 10 method, and the 10-fold baseline method. Each approach tries to mark the same clinical reality, but they do so differently. One favors speed and simplicity, while the others draw on historical patterns and retrospective smoothing that can sharpen precision after the fact.
RSV seasonality is never perfectly uniform across the United States. Regional variation matters because a national estimate can blur local timing, and a prevention strategy that is on time in one part of the country may arrive too late in another. This analysis focused on those regional differences and assessed not only onset, but also season duration and the share of annual detections captured inside the epidemic window. Those are the practical measures that determine whether a season definition is merely tidy on paper or actually useful in surveillance practice.
Why the 3% Threshold Stayed Clinically Useful
The 3% threshold generally identified epidemic onset later than the slope 10 method. It also varied by region when compared with the 10-fold baseline method. In plain surveillance terms, the 3% rule was more conservative at the start of the season, while the retrospective slope approach tended to pull the onset forward. A later onset can be a drawback if the goal is to describe the earliest rise in transmission, but it can also reduce the risk of calling a season too early on the basis of noise, especially when laboratory testing patterns fluctuate from year to year.
One sentence stands out from the analysis: RS10 captured the highest percentage of detections. That matters because a season definition that misses a substantial share of positive tests loses clinical and public health value, even if it appears elegant in retrospect. RS10 also varied least annually, suggesting a more stable signal across surveillance years. The method’s limitation is straightforward, though. It can only be applied retrospectively, which makes it less useful for near real-time operational decisions about when to begin seasonal prevention messaging or timing of product administration.
The 3% threshold produced the shortest epidemic duration, while RS10 produced the longest. A shorter epidemic window may be desirable if the aim is to define a clearly bounded season, but it may also fail to capture the full rise and fall of transmission in years when RSV circulation begins gradually or lingers into the shoulder months. The balance between sensitivity and practicality is the central issue here. Surveillance systems have to work in real time, not only in hindsight, and the best method is the one that remains interpretable when prevention decisions are still being made.
Pandemic Shifts Did Not Break the Simplest Method
Before the COVID-19 pandemic, the 3% threshold and the 10-fold baseline method showed similar percentages of detections captured within the epidemic period. After the pandemic, detections decreased with the 10-fold baseline approach. The finding suggests that historical baselines can be distorted when a respiratory virus experiences an unusual disruption in circulation. When population immunity, contact patterns, testing behavior, and healthcare-seeking all shift at once, a method anchored too tightly to prior seasons may lose fidelity.
By contrast, the 3% threshold appears to have remained stable in the postpandemic environment. The analysis did not find evidence that this simpler method was undermined by the pandemic-era disruption in a way that would make it unreliable for defining RSV seasonality. For surveillance programs, simplicity is not a weakness when it preserves consistency across seasons and regions. A near real-time threshold can be repeated, communicated, and operationalized with less delay than a retrospective model that requires the season to finish before it can be applied.
What the Three Methods Mean for Prevention Timing
The comparison between these approaches is more than a statistical exercise. It affects when clinicians, health systems, and public health programs can anticipate RSV circulation and align prevention efforts. An earlier onset identified by RS10 may reflect a more complete retrospective capture of the season, but it does not translate well into a live operational framework. The 10-fold baseline method did not consistently identify season onset earlier across regions, which weakens its appeal as a universal timing tool. Regional heterogeneity remains a real constraint, and no single method fully erases it.
The 3% threshold survives because it is simple, transparent, and usable as the season is unfolding. Its conservatism may even be an advantage when the purpose is to mark the beginning of an epidemic period without relying on delayed analytic reconstruction. In a surveillance system that feeds prevention decisions, that simplicity has direct value. It lets users apply the same rule across settings without waiting for a retrospective recalibration each year.
A key operational lesson comes from the mix of results: methods that maximize completeness after the season ends are not automatically the best tools for current-season decision-making. RS10 appears strong for retrospective characterization, but it cannot guide live planning. The 3% threshold may not catch the earliest inflection point in every region, yet it performed consistently enough to remain a preferred U.S. approach for near real-time RSV seasonality assessment.
Regional Variation Still Sets the Boundaries
The study’s regional comparisons reinforce an old epidemiologic truth. RSV does not respect a single calendar. Timing shifts by geography, and surveillance tools must account for those differences if they are to support timely prevention. The 3% threshold varied by region when judged against the 10-fold baseline method, which means local interpretation still matters even when a national rule is available.
The broader implication is not that one method replaces the others outright. Rather, each analytic approach serves a different purpose. RS10 offers strong retrospective coverage and early onset detection in a historical sense. The 10-fold baseline method provides another benchmark but did not consistently improve onset timing across regions. The 3% threshold remains attractive because it is simple enough for routine use and robust enough to withstand a postpandemic shift in RSV circulation.
For clinicians and public health teams, the practical question is whether a surveillance definition can be trusted when decisions still need to be made. This analysis suggests that the answer for the 3% threshold is yes, even after a period when routine respiratory virus patterns were disrupted. The evidence also leaves an important boundary in place: retrospective methods may describe the epidemic more completely, but they are not built for the same day-to-day decisions that near real-time monitoring requires.