In the United States, flu season runs from October through May, with activity most often peaking in February. Over the past four decades of CDC tracking, February has been the peak month more than any other, topping out in 18 of 42 tracked seasons. December and January are the next most common peaks, but the virus can surge as early as October or linger into April.
When Flu Activity Peaks
The CDC has tracked the peak month of flu activity for every season from 1982-1983 through 2024-2025. The pattern is clear but not perfectly predictable. February leads with 18 peak seasons, followed by December (9 seasons), January (6 seasons), and March (6 seasons). Only one season peaked as early as October, one peaked in November, and one peaked as late as April. No season has ever peaked in May.
What this means practically: flu cases typically start climbing in late fall, build through December and January, and hit their highest point sometime between December and March. Activity then tapers through spring. The 2020-2021 season was the sole exception in the dataset, with no discernible peak at all due to the unusually low circulation of influenza that year.
Why Flu Thrives in Winter
The seasonal pattern isn’t a coincidence. Research published in the Proceedings of the National Academy of Sciences found that absolute humidity, the total amount of moisture in the air, is the strongest environmental predictor of how well the flu virus survives and spreads. In winter, absolute humidity drops significantly both indoors and outdoors in temperate climates. That dry air allows the virus to remain infectious longer on surfaces and in airborne droplets, and it transmits more efficiently between people.
Absolute humidity alone explains about 50% of the variation in flu transmission and 90% of the variation in how long the virus survives outside a host. Relative humidity, the percentage you see on weather apps, is a much weaker predictor. The relationship is also nonlinear, meaning small drops in humidity at already-low levels can cause disproportionately large jumps in virus survival. This is why flu doesn’t gradually increase as fall arrives but tends to spike sharply during the coldest, driest stretches of winter.
Flu Season in the Southern Hemisphere
If you live in or plan to travel to countries south of the equator, the calendar flips. Flu season in the Southern Hemisphere typically runs from April through September, sometimes extending into October or November. This roughly mirrors the Northern Hemisphere’s October-through-May window, shifted by six months to align with the Southern Hemisphere’s winter. Public health officials often look at Southern Hemisphere flu patterns for clues about which strains may dominate the upcoming Northern Hemisphere season.
Best Time to Get the Flu Shot
The CDC recommends getting vaccinated in September or October for most people. Your body needs about two weeks after vaccination to build full antibody protection, so a late September shot means you’re covered by mid-October, right as cases begin to rise.
Getting vaccinated too early can be a problem. July and August vaccination is not recommended for most groups because vaccine-induced immunity fades over time, and an early shot may leave you less protected during the peak months of January and February. Research from a large analysis of adult flu vaccine effectiveness found that protection drops by roughly 9% every 28 days starting about six weeks after vaccination. That means someone vaccinated in late July could see meaningfully reduced protection by February, exactly when they need it most.
This waning effect is especially relevant for older adults. People 65 and older showed the steepest decline in protection against certain flu strains over time. For this group, timing the shot to September or October is particularly important to maintain protection through the peak months. That said, getting vaccinated after October is still worthwhile. A late shot beats no shot, and the flu can circulate well into spring.
How Long Flu Season Lasts
From the first uptick in cases to the last meaningful wave, flu season spans roughly seven to eight months. Activity usually becomes detectable in October, accelerates through the holiday season, peaks between December and March, and fades by late April or May. In some years, a secondary wave of a different flu strain can extend meaningful activity into spring.
The total window matters for planning. If you missed your flu shot in October and it’s now January, you still have weeks or months of circulating virus ahead of you. Vaccination at that point still provides protection during whatever remains of the season, since antibodies develop within two weeks of receiving the shot.