When is the influenza A season?

Influenza A Season Timing

In the United States, influenza A season typically occurs from October through May in the Northern Hemisphere, with peak activity most commonly occurring in February, though the timing can vary each year. 1

Seasonal Patterns in the Northern Hemisphere

• Influenza activity typically begins as early as October
• Peak activity most commonly occurs in February (42% of seasons) 1
• In 75% of influenza seasons from 1982-2018, peak activity occurred in January or later 1
• In 58% of seasons, the peak was in February or later 1
• The season can extend well into March and beyond, sometimes lasting until May 1

Regional and Global Variations

• Northern Hemisphere: October through May (peak December-March) 2
• Southern Hemisphere: April through September 1, 2
• Tropics: Influenza can circulate year-round with less predictable seasonality 1, 2

Multiple Peaks and Variations

The influenza season can have multiple peaks within the same season 1. For example:

• The 2018-2019 season was unusually long, lasting 21 consecutive weeks 1
• The 2019-2020 season had three distinct peaks of activity 1
• The 2020-2021 season was unusually mild due to COVID-19 mitigation measures 1

Implications for Vaccination Timing

Based on these seasonal patterns, the CDC and AAP recommend:

• Vaccination should be completed by the end of October 1
• Children requiring two doses should start vaccination as soon as vaccine becomes available 1
• Vaccination efforts should continue throughout the entire influenza season, even after influenza activity has been documented in a community 1
• Very early vaccination (July or August) may be associated with suboptimal immunity before the end of the season, particularly in older adults 1

Travel Considerations

• Travelers to the Southern Hemisphere during April-September should consider influenza vaccination before departure 1, 2
• Travelers to tropical regions should consider vaccination at any time of year 2
• Those traveling with tour groups that include people from areas where influenza circulates during April-September should consider vaccination 3

Monitoring and Prediction Challenges

It's important to note that predicting the exact timing, duration, or severity of the influenza season with accuracy is impossible 1. The onset, peak, and decline of influenza activity varies from year to year, making continuous surveillance and preparedness essential throughout the typical season months.

The unpredictability of influenza seasonality underscores the importance of timely vaccination and maintaining vigilance for influenza cases throughout the entire season, including late-season cases which can sometimes be associated with higher morbidity 4.