February is the most common peak month for flu activity in the United States, based on decades of CDC surveillance data. Most flu seasons peak somewhere between December and February, but February has historically been the single month where the highest percentage of flu tests come back positive. The 2024-25 season followed this pattern exactly, peaking in early February 2025.
When Flu Season Peaks
Flu season runs through fall and winter, but the worst stretch is a narrower window than most people assume. Activity typically starts climbing in November, hits its highest point between December and February, and tapers off by late March or April. Within that range, the specific peak shifts from year to year.
Recent seasons show how much the timing can vary. The 2022-23 season peaked unusually early, in mid-November. The following year, 2023-24, peaked in late December. And the 2024-25 season peaked in early February, which the CDC noted was more consistent with pre-COVID seasonal patterns. So while February is the most frequent peak month historically, any given year might surprise you with a December or even November surge.
Why Winter Makes Flu Worse
Cold weather alone doesn’t explain flu seasonality. The real driver is something called absolute humidity: the total amount of moisture in the air. A landmark study published in the Proceedings of the National Academy of Sciences found that absolute humidity accounts for 90% of the variation in how long flu virus particles survive outside the body, and 50% of the variation in how easily the virus transmits between people. Temperature and relative humidity were far weaker predictors.
In winter, absolute humidity drops to its lowest levels both outdoors and inside heated buildings. That dry air lets flu virus particles hang in the air longer and remain infectious. It’s not just that people crowd indoors during cold months. The air itself becomes a better vehicle for spreading the virus. This seasonal humidity cycle bottoms out in the coldest winter months, which lines up neatly with the December-through-February peak window.
How Recent Seasons Compare
The three most recent flu seasons illustrate how unpredictable peak timing can be, even within the expected winter window. The 2022-23 season was notably aggressive and early, with emergency department visits for flu peaking at 6.7% of all visits during the week ending November 26. The 2023-24 season peaked a month later, in late December, at a lower intensity of 5.8%. The 2024-25 season built more slowly, starting its climb in mid-November and not reaching its highest point until early February, but it hit harder than the previous two seasons.
The takeaway: you can’t count on the worst week falling on the same date each year. Planning around the full December-to-February danger zone gives you a more realistic picture.
Best Time to Get Vaccinated
Since flu activity most often peaks in February, getting vaccinated in September or October gives your immune system time to build protection before the worst months arrive. The CDC recommends that most people be vaccinated by the end of October. Getting the shot too early, in July or August, can be counterproductive because protection fades over time, especially for adults 65 and older and people in early pregnancy.
If you miss October, getting vaccinated in November or even December still offers meaningful protection. The vaccine takes about two weeks to reach full effectiveness, so a late-November shot would have you covered well before a typical February peak.
Flu Season in the Southern Hemisphere
If you’re traveling internationally, flu season flips with the seasons. In the Southern Hemisphere, flu activity runs from roughly April through September, sometimes extending into October or November. Countries like Australia, Brazil, and South Africa experience their peak during the months that are summer in the Northern Hemisphere. Public health officials often watch the Southern Hemisphere’s flu season for clues about which strains and severity levels the U.S. might face a few months later.