Flu season in the United States typically runs from October through March, with activity peaking between December and February. That’s roughly six months of elevated risk, though the exact timing shifts from year to year. The CDC formally tracks influenza from epidemiologic week 40 (early October) through week 39 of the following year, covering a full 52-week surveillance cycle, but the months that matter most for your health fall within a much narrower window.
When Flu Season Starts and Peaks
Flu cases begin climbing in the fall, usually by October or November, as temperatures drop and people spend more time indoors. The peak, when the largest number of people are getting sick at once, most often lands between December and February. Some seasons peak as early as December, others as late as March. This variability makes it difficult to predict exactly when the worst week will hit in any given year.
In temperate climates like the U.S., the peak season is typically contained within a one-to-three-month window, with low variation from year to year. Research comparing flu patterns across different climate zones found that temperate locations show regular, annual winter epidemics with peak timing varying by only about four to eight weeks from one season to the next.
Why Cold Weather Drives Flu Transmission
Flu season isn’t just a calendar convention. It’s driven by environmental conditions that help the virus spread. A well-known study using guinea pigs as a transmission model found that influenza spreads most efficiently in cold, dry air. At low relative humidity (20% to 35%), transmission was highly efficient, infecting nearly every exposed animal. At 80% humidity, transmission was completely blocked.
Temperature plays an equally important role. At 5°C (41°F), the virus spread at much higher rates than at 20°C (68°F). At 30°C (86°F), no transmission occurred at all, even when humidity was low. Cold air also holds less moisture, so winter naturally combines the two conditions that favor the virus: low temperatures and low humidity. This is why flu season aligns so closely with the coldest, driest months of the year.
How Long Individual Seasons Last
The total duration of a flu season, measured from when cases rise above baseline levels to when they fall back down, ranges from about two to four months of significant activity. Research analyzing multiple seasons in temperate locations found that individual seasons lasted between 57 and 117 days, or roughly two to four months. That doesn’t mean you can’t catch the flu outside that window, but the overwhelming majority of cases cluster within it.
Influenza viruses circulate year-round at low levels. You can technically get the flu in July, but the odds are far lower than in January. The seasonal concentration of cases is what creates the surge that strains hospitals and drives public health campaigns.
Flu Season in the Southern Hemisphere and Tropics
If you live in or travel to the Southern Hemisphere, flu season is flipped. In Australia, influenza typically peaks between May and September, corresponding to their winter months. The same cold, dry conditions that drive flu in North America apply, just six months offset.
Tropical regions are a different story entirely. In countries like Vietnam, flu doesn’t follow a neat winter pattern. Peaks can occur throughout the calendar year, and the timing is far less predictable. Research found that peak flu season in tropical Vietnamese regions would have to be defined as a six-to-seven-month period to capture the variability, compared to the tight one-to-three-month peak window in temperate climates. The variation in peak timing was two to three times greater than in temperate locations, making it harder for public health officials to plan vaccination campaigns.
Best Timing for a Flu Vaccine
Knowing when flu season peaks helps you time your vaccination. The CDC recommends getting vaccinated by the end of October for most people, which positions your immune response to be strongest during the December-to-February peak. Vaccine protection declines over time, which is one reason a new shot is needed every year. Adults 65 and older, along with pregnant women in their first or second trimester, are generally advised not to get vaccinated too early in July or August, since their protection may wane before the season’s worst weeks arrive.
If you miss the October window, getting vaccinated later still helps. Flu activity can persist into March or even April in some years, so a January or February vaccination still provides meaningful coverage for the tail end of the season. The key is matching your peak immunity to the period of highest community transmission, and for most of the U.S., that means the dead of winter.