From the Guidelines
Influenza can occur year-round in temperate climates, but it typically follows a seasonal pattern with epidemics occurring during winter months. According to the most recent study 1, peak influenza activity in the United States typically occurs from January through March, but influenza viruses can begin to circulate in early fall (October) and continue to late spring (May or later). This study highlights that predicting the onset and duration of the influenza season with accuracy is impossible, and timely influenza vaccination is important to ensure that individuals are optimally protected before influenza viruses are circulating in the community.
Key Points
- Influenza activity in the United States is typically low during the summer, but cases and outbreaks can occur, particularly among international travelers who may be exposed to influenza year-round, depending on destination 1.
- Influenza can occur throughout the year in the tropics, and individuals traveling to parts of the world where influenza activity is ongoing should consider seasonal influenza vaccination 2 weeks before departure, if available 1.
- The American Academy of Pediatrics (AAP) and the Centers for Disease Control and Prevention (CDC) recommend that children, especially those who need 2 doses, should be immunized as soon as a vaccine becomes available and complete influenza vaccination by the end of October 1.
Seasonality and Vaccination
The seasonality of influenza is influenced by several factors, including indoor crowding during colder months, lower humidity, reduced vitamin D levels, and environmental conditions that enhance viral stability and transmission. Annual vaccination timed before the local flu season begins remains the most effective preventive measure, regardless of when influenza activity occurs. The study 1 suggests that immunity after influenza vaccination can wane over time, but the AAP and CDC recommend continuing to vaccinate individuals as long as influenza viruses are circulating and an unexpired vaccine is available.
From the Research
Influenza Epidemics in Temperate Climates
- Influenza infections exhibit a strong seasonal cycle in temperate regions, with epidemics often occurring during the winter months 2, 3.
- The seasonal pattern of influenza in temperate climates is thought to be driven by low specific humidity conditions, which facilitate the airborne survival and transmission of the influenza virus 2.
- However, there is evidence to suggest that influenza can occur year-round in some temperate climates, particularly in areas where the climate is mild and humid 4.
Factors Influencing Influenza Seasonality
- Specific humidity and temperature are key factors influencing influenza seasonality, with epidemics often occurring when these conditions are at minimal levels 2.
- Precipitation and rainfall can also play a role in influenza seasonality, particularly in tropical and subtropical regions where epidemics often occur during the rainy season 2, 4.
- The physiologic effects of cold weather and dry air on the upper respiratory system may also contribute to immune dysfunction and increased susceptibility to flu-causing pathogens 3.
Global Influenza Seasonality
- Influenza seasonality patterns vary globally, with different regions experiencing epidemics at different times of the year 4.
- Tropical regions often experience year-round influenza activity, while temperate regions typically experience seasonal epidemics during the winter months 2, 4.
- Further research is needed to understand the complex interactions between environmental stimuli, mediating mechanisms, and the seasonal timing of influenza epidemics, particularly in tropical localities 4.