Flu season in the United States typically lasts about five to six months, running from late fall through early spring. Activity usually picks up in November, peaks between December and February, and tapers off by March or April. The exact timing shifts from year to year, but that general window holds remarkably consistent.
When Flu Season Starts and Peaks
Influenza viruses circulate year-round at low levels, but meaningful seasonal activity begins in mid to late November most years. During the 2024-2025 season, for example, flu activity started climbing in mid-November and outpatient visits for flu-like illness stayed above baseline levels for 17 consecutive weeks, from November through March. The peak hit during the week ending February 8, 2025, when nearly 8% of all outpatient visits were for flu-like symptoms.
December through February is consistently the highest-risk window. That three-month stretch is when you’re most likely to catch the flu, encounter it at work or school, and see the highest hospitalization numbers. Some seasons peak as early as December, others as late as March, but February is the single most common peak month.
Why Flu Thrives in Winter
Cold weather alone doesn’t cause the flu, but the conditions that come with winter create an ideal environment for the virus. Research published in the Proceedings of the National Academy of Sciences found that absolute humidity, the actual amount of water vapor in the air, is the key factor. When absolute humidity drops in winter, the influenza virus survives longer on surfaces and in the air, and it transmits between people more efficiently.
This matters more than temperature or relative humidity. Winter air holds far less moisture both outdoors and inside heated buildings, and that dry environment lets viral particles stay infectious longer after someone coughs or sneezes. It’s one reason flu season aligns so tightly with the coldest, driest months of the year in temperate climates.
Flu Season in the Southern Hemisphere
If you’re traveling internationally or live in the Southern Hemisphere, the calendar flips. Countries like Australia, Argentina, and South Africa typically see flu activity between April and September, sometimes extending into October or November. The pattern follows the same logic: their winter months bring low humidity and colder temperatures, creating the same conditions that drive transmission in the Northern Hemisphere six months earlier.
Public health officials actually watch the Southern Hemisphere’s flu season closely for clues about what the upcoming Northern Hemisphere season might look like, including which strains are circulating and how severe the season could be.
How Long You’re Contagious With the Flu
Beyond the broader season, it helps to know your personal contagious window. You can spread influenza starting about one day before your symptoms appear and for five to seven days after you get sick. The first three days of illness are when you’re shedding the most virus and pose the greatest risk to people around you.
Young children and people with weakened immune systems can remain contagious for longer than a week. That pre-symptomatic day of contagiousness is part of what makes flu so hard to contain: you can pass it to coworkers, family members, or classmates before you even know you’re sick.
Best Time to Get Vaccinated
Timing your flu shot matters more than most people realize. Vaccine protection doesn’t last indefinitely. A large analysis covering nine pre-COVID flu seasons in Ontario found that flu vaccine effectiveness in adults drops by about 9% every 28 days starting roughly six weeks after vaccination. That means getting your shot too early in the fall could leave you with reduced protection during the February peak.
For most people, September or October is the sweet spot. That timing gives your immune system about two weeks to build full protection before activity ramps up in November, and it keeps your immunity strongest through the December-to-February peak. If you get vaccinated in early September, your protection will have declined noticeably by March, but you’ll still be well-covered during the months that matter most. Getting vaccinated later, even into January, still offers benefit if you haven’t been sick yet that season.
Why Some Flu Seasons Last Longer Than Others
Not every flu season follows the same neat timeline. Several factors can stretch or compress the active window. When multiple influenza strains circulate in waves rather than all at once, the season can drag on longer. Influenza A strains often dominate earlier in the season, while influenza B strains sometimes peak later, effectively extending the tail end of flu activity into April or even May.
A mild winter with unusual warm spells can delay the peak. A particularly harsh strain that the population has little immunity against can cause activity to spike earlier and more sharply. The 2024-2025 season fit a fairly typical pattern with a February peak, but other recent seasons have peaked as late as March. The unpredictability is part of why public health agencies track flu activity on a weekly basis rather than relying on fixed calendar dates.