When Flu Season Starts in the USA
Flu season in the United States typically begins in October, with activity increasing through November and peaking between late December and February 1.
Typical Seasonal Pattern
Based on the most recent comprehensive guideline data, the influenza season follows a predictable pattern:
- October: Localized outbreaks begin, marking the start of seasonal activity 2
- November-December: Activity increases substantially 1
- Late December-February: Peak activity occurs in most seasons 3, 4
- March-May: Activity tapers off, though can remain elevated 1
Historical data from 1982-2015 shows that in 74% of influenza seasons, peak activity did not occur until January or later, and in 59% of seasons, the peak was in February or later 2.
Recent Variations and Important Caveats
The 2022-2023 season demonstrated that flu can start earlier than typical, with influenza activity beginning in late September and October (earlier than prior seasons) 1. This earlier onset was particularly notable in the Southeast, with high pediatric hospitalization rates occurring as early as November 5.
The 2024-2025 season officially began tracking on October 1,2024, and was characterized as a high-severity season 1.
Clinical Implications for Vaccination Timing
Vaccination should ideally occur during September or October 6. However, the guidelines emphasize several critical timing considerations:
Children requiring 2 doses (aged 6 months-8 years without prior vaccination history): Should receive their first dose as early as possible, including July-August if vaccine is available, to permit the second dose (≥4 weeks later) by end of October 6
Most adults and pregnant women in first/second trimester: Should avoid July-August vaccination due to potential waning immunity, unless there's concern later vaccination won't be possible 6
Pregnant women in third trimester during July-August: Can consider early vaccination to protect infants after birth 6
Vaccination should continue throughout the entire influenza season, even after community activity begins, as long as unexpired vaccine is available 2, 6. This is critical because the duration and timing of influenza seasons vary unpredictably, and delaying vaccination risks missed opportunities.
Key Takeaway for Practice
While October marks the traditional start of flu season, providers should maintain vigilance for earlier onset (as occurred in 2022-2023) and ensure vaccination programs are operational by September to capture optimal timing while avoiding the pitfall of waiting too long and missing the window before community transmission begins 1, 6.