RSV Season Timing in the United States
In most of the continental United States, RSV season typically begins in October or November, peaks in December through early February, and ends by March through early May 1.
National Patterns
Based on surveillance data from 2007-2013, the RSV season across most U.S. regions demonstrates consistent patterns 1:
- Season onset: October to November in most regions
- Peak activity: Mid-December to early February
- Season offset: March to early May
- Duration: 13-23 weeks (median approximately 20 weeks)
More recent surveillance from 2017-2023 confirms that prepandemic seasons (2017-2020) began in October, peaked in December, and ended in April 2. The RSV season is defined as weeks when ≥3% of PCR tests are positive for RSV 2.
Regional Variations
Florida
Florida has distinctly different RSV seasonality compared to the rest of the United States 1:
- Earlier onset (can begin as early as July)
- Longer duration
- Less predictable patterns
- Requires consultation of Florida Department of Health data for timing prophylaxis
Alaska
Alaska experiences 1:
- Less predictable seasonality
- Longer RSV season duration than national average
- Should use state-generated RSV surveillance data
Geographic Gradient
RSV activity follows a geographic pattern 2, 3:
- Earliest onset: Florida and Southeast regions
- Latest onset: Northern and Western regions
- This north-to-south, east-to-west gradient is consistent across seasons
Clinical Implications for Prophylaxis Timing
For immunoprophylaxis administration 4:
- Most continental U.S.: Begin administration in October through March
- Infants born during season: Administer within 1 week of birth
- Optimal timing: Shortly before RSV season onset
- Only one dose of nirsevimab is needed per season
Important Caveats
Post-COVID-19 pandemic disruption: RSV seasonality was significantly altered during 2020-2022 2:
- The typical 2020-21 winter epidemic did not occur
- The 2021-22 season began unusually early (May), peaked in July, and ended in January
- The 2022-23 season showed patterns returning toward prepandemic norms but with some variation
- Clinicians should remain aware that off-season RSV circulation may continue
Tropical climates (southern Florida, Hawaii, Puerto Rico, Guam, U.S.-affiliated Pacific Islands, U.S. Virgin Islands) have different or unpredictable seasonality and require consultation of local/territorial guidance 4.
Sporadic cases occur outside the main season in many areas, but maximum benefit from prophylaxis occurs during peak season, not when incidence is low 1.