From the Guidelines
Influenza season in the United States typically runs from October to May, with peak activity usually occurring between December and February. The timing can vary by region, with some areas experiencing earlier or later peaks [ 1 ]. Southern Hemisphere countries generally have their flu season during their winter months (April to September). The seasonal pattern occurs because influenza viruses survive better in cold, dry conditions and people tend to spend more time indoors in close contact during winter months, facilitating transmission.
Key Considerations
- The CDC recommends getting vaccinated by the end of October, though vaccination even later can still be beneficial [ 1 ].
- During flu season, practicing good hygiene, avoiding close contact with sick individuals, and staying home when ill can help prevent the spread of influenza.
- Vaccination efforts should continue throughout the season because the duration of the influenza season varies, and influenza activity might not occur in certain communities until February, March, or later [ 1 ].
Regional Variations
- The timing of influenza season can vary by region, with some areas experiencing earlier or later peaks [ 1 ].
- In the United States, localized outbreaks that indicate the start of seasonal influenza activity can occur as early as October [ 1 ].
Vaccination Timing
- Vaccination should be offered by the end of October, with children aged 6 months through 8 years who require 2 doses receiving their first dose as soon as possible after the vaccine becomes available [ 1 ].
- Community vaccination programs should balance maximizing likelihood of persistence of vaccine-induced protection through the season with avoiding missed opportunities to vaccinate or vaccinating after onset of influenza circulation occurs [ 1 ].
From the Research
Influenza Season
- The influenza season typically begins in October and lasts until March in the northern hemisphere 2, 3, 4, 5.
- In the southern hemisphere, the influenza season typically occurs from April to September 3, 4.
- The seasonality of influenza can vary significantly in tropical regions, with the potential for year-round transmission 3, 4.
- The exact timing of the influenza season can impact the effectiveness of influenza vaccination, particularly for travelers between the northern and southern hemispheres 3, 4.
Regional Variations
- In temperate climates, the peak influenza season typically occurs between October and April in the northern hemisphere and between April and October in the southern hemisphere 3, 4.
- The influenza season in the United States typically starts in October and can last until May, with the peak activity often occurring in mid-March 5.
- The composition of influenza vaccines may vary between the northern and southern hemispheres, and between different regions, due to differences in circulating influenza strains 3, 6.
Travel-Related Considerations
- Travelers between the northern and southern hemispheres may be at risk of influenza infection during their travels, particularly if they have not received the recommended influenza vaccine for their destination 3, 4.
- Health care providers should be aware of influenza activity and provide guidance on prevention, diagnosis, and treatment options for travelers 4.