In the United States and most of the Northern Hemisphere, flu season runs from October through May, with activity peaking most often in February. Over the past 40 years of CDC tracking, February has been the peak month in 18 out of 43 seasons. December, January, and March each account for six or seven peak seasons. That means the worst of it typically lands somewhere between December and March, but the virus circulates for months on either side of that window.
When Flu Season Peaks in the U.S.
Flu cases usually start climbing in October and November, build through December and January, and hit their highest point in February. Activity then tapers off through March and April, though it can linger into May in some years. The exact timing shifts from season to season. Some years see an early December surge, while others don’t peak until March.
This variability matters for planning. You can’t circle one week on the calendar and call it “flu season.” It’s better understood as a five-to-seven-month stretch where risk is elevated, with a roughly three-month core (December through February) when you’re most likely to catch it.
Flu Season in the Southern Hemisphere
Countries south of the equator experience flu season on an opposite schedule: April through September, sometimes extending into October or November. Australia, South Africa, Argentina, and Chile all follow this pattern. Because their winter comes first, public health officials in the Northern Hemisphere watch Southern Hemisphere flu data for early clues about which strains might dominate later in the year and how severe the upcoming season could be.
Tropical Regions Don’t Follow the Same Rules
Near the equator, the neat winter-flu pattern breaks down. Countries like Singapore, Malaysia, Indonesia, Vietnam, Nigeria, Kenya, and Uganda show flu activity year-round, often with multiple peaks instead of one clear season. This makes vaccination timing more complicated in tropical climates and is one reason global flu surveillance looks very different depending on latitude.
Why Flu Thrives in Winter
Several factors converge in cold months to give the virus an advantage. The most well-supported explanation involves humidity. When the air holds less moisture (low absolute humidity, typical of cold winter air), the influenza virus survives longer on surfaces and in airborne droplets. Research published in PNAS found that this improved virus survival, not changes in droplet size or how far they travel, is the primary way dry air boosts transmission.
Other contributing factors include people spending more time indoors in close proximity, reduced UV radiation that would otherwise degrade the virus outdoors, and possible seasonal dips in immune function related to lower vitamin D levels. None of these factors alone fully explains the pattern, but together they create conditions where flu spreads efficiently from person to person.
Best Time to Get Vaccinated
September and October are the ideal months to get your flu shot. The vaccine takes about two weeks to build full protection, so getting vaccinated in this window means you’re covered before activity ramps up in November and December.
Getting vaccinated too early can be a problem. July and August vaccinations are generally not recommended because vaccine-induced immunity can wane over the course of a long season, particularly in older adults. If you got your shot in late July, your protection might be noticeably weaker by February or March, right when the virus is most likely peaking.
That said, a late vaccine is better than no vaccine. If October passes and you haven’t been vaccinated, it’s still worth getting a shot any time flu is circulating. Even a February vaccination provides some benefit if the season runs late.
How Seasons Have Shifted Recently
The COVID-19 pandemic disrupted normal flu patterns significantly. The 2020-2021 flu season was essentially nonexistent, suppressed by masking, social distancing, and travel restrictions. When flu returned, its timing was less predictable for a couple of years. More recent seasons (2023-2024 and 2024-2025) have shown flu arriving more in sync with other respiratory viruses like RSV, creating overlapping waves of illness that strain hospitals during the same winter months.
These shifts reinforce the value of getting vaccinated on the standard September-October timeline rather than trying to guess exactly when the peak will hit in any given year. The virus has returned to a broadly predictable winter pattern, but the specific peak month remains impossible to forecast in advance.