RSV Infection Peak Timing
RSV infections in the United States typically peak in January or February during the winter season. 1
Seasonal Pattern in the United States
The RSV season follows a predictable pattern across most of the continental United States:
- Season onset usually begins in November or December 2, 1
- Peak activity occurs in January or February 2, 1, 3
- Season conclusion typically ends by late March or sometime in April 2, 1
- The median season duration is 15 weeks nationally, with regional variation ranging from 13 to 23 weeks 1
Geographic Variation Within the United States
The timing of RSV peaks varies by geographic region due to a clear north-to-south gradient:
- Southern United States experiences the earliest onset of RSV activity 1
- Midwestern states tend to experience the latest onset, with median onset at week 1 of the following year 1, 4
- Western and Northeast regions have seasonal timing that falls between the South and Midwest patterns 1
- Florida shows substantial variation and requires separate consideration, with an earlier season onset and longer duration than the rest of the country 1, 5, 6
International Patterns
- In Portugal and Europe, the RSV season typically starts in December, with peak activity occurring during a median epidemic duration of 10 to 11 weeks, though it can extend from October/November to April/May 2
- In the Northern Hemisphere broadly, RSV circulates predominantly between November and March 1
Clinical Implications for Prophylaxis Timing
Understanding peak timing is critical for optimal prevention strategies:
- The first dose of palivizumab should be administered before the RSV season starts, typically in November, to provide protection when exposure risk is highest 1, 7
- Monthly dosing of palivizumab provides more than 20 weeks of protective antibody concentrations, covering most seasonal variation 1
- Prophylaxis provides the greatest benefit during peak season (January-February), not during periods of low RSV incidence 1
Important Caveats
- Year-to-year variation occurs within the same community and between communities in the same region, even though the overall pattern remains predictable 1, 4
- The COVID-19 pandemic disrupted RSV seasonality during 2020-2022, with the 2021-22 season beginning in May and peaking in July, though patterns are returning toward prepandemic timing 3
- Clinicians should maintain awareness that off-season RSV circulation might continue despite the return toward typical seasonal patterns 3