When does Respiratory Syncytial Virus (RSV) peak?

When Does RSV Peak?

RSV infections in the United States typically peak in mid-December through early February, with the season generally running from November through April in most regions. 1

National Seasonal Pattern

The timing of RSV circulation follows a predictable pattern in the continental United States:

• Season onset: RSV activity typically begins in November or December 1
• Peak activity: The highest incidence occurs in January or February 1
• Season conclusion: RSV activity typically ends by late March or sometime in April 1
• Median duration: The national median season duration is 15 weeks, though regional variation ranges from 13 to 23 weeks 2, 1

The FDA-approved palivizumab label confirms that "in the northern hemisphere, the RSV season typically commences in November and lasts through April, but it may begin earlier or persist later in certain communities." 3

Geographic Variation Within the United States

RSV timing follows a clear north-to-south gradient, with southern states experiencing earlier onset:

• Southern United States: Experiences the earliest onset of RSV activity 1
• Midwestern states: Tend to experience the latest onset 1
• Western and Northeast regions: Fall between the South and Midwest patterns 1

Special Regional Considerations

Florida shows substantial variation across different regions within the state, requiring clinicians to consult local Florida Department of Health surveillance data for optimal timing 2, 1

Alaska experiences higher RSV hospitalization rates and a longer RSV season, particularly in remote regions and among Alaska Native populations 2, 1

Recent Post-Pandemic Patterns

The COVID-19 pandemic significantly disrupted typical RSV seasonality:

• 2020-21 season: The typical winter RSV epidemic did not occur 4
• 2021-22 season: An atypical season began in May, peaked in July, and ended in January 4
• 2022-23 season: Started in June and peaked in November—later than 2021-22 but earlier than prepandemic seasons 4
• Current trend: The 2022-23 pattern suggests seasonal patterns are returning toward prepandemic norms, though clinicians should remain aware that off-season circulation may continue 2, 4

International Patterns

In the Northern Hemisphere broadly, RSV circulates predominantly between November and March 1. In Portugal specifically, the RSV season typically starts in December with a median epidemic duration of 10-11 weeks, though it can extend from October/November to April/May 2

Clinical Implications for Prophylaxis Timing

The timing of RSV prevention strategies must align with local seasonal patterns:

• Palivizumab: The first dose should be administered before the RSV season starts, typically in November, to provide protection when exposure risk is highest 1, 3
• Maternal RSVpreF vaccination: The CDC recommends administration during September through January in most of the continental United States 1
• Maximum benefit: Prophylaxis provides the greatest benefit during peak season, not during periods of low RSV incidence 2, 1

Important Caveats

Year-to-year variation occurs even within the same community, despite overall predictable patterns 1. Clinicians should maintain awareness of local surveillance data, as the exact timing can shift between years and differs between communities in the same region 2, 1. A small number of sporadic RSV hospitalizations occur before or after the main season in many areas, but maximum benefit from prophylaxis is derived during the peak rather than when incidence is low 2