In the United States, flu season runs roughly from October through March, with activity peaking between December and February. That’s about six months of elevated risk, though the exact timing shifts from year to year. Influenza viruses circulate year-round, but the combination of cold air and dry indoor heating creates a seasonal window when transmission surges.
The U.S. Flu Season Month by Month
Flu cases start climbing in the fall, usually becoming noticeable by October or November. The heaviest stretch falls between December and February, when hospitals see the highest volume of flu-related visits. Activity then tapers through March and April, though late-season waves occasionally push significant cases into May.
The peak week varies from season to season. Some years, flu hits hardest in late December. Others don’t peak until February or even early March. This unpredictability is one reason public health officials frame the season broadly rather than pinning it to a single month.
Why Winter Drives Flu Transmission
Cold, dry conditions are the engine behind flu season. Research using animal models found that influenza spreads most efficiently at low relative humidity levels of 20% to 35%, which is typical of heated indoor air during winter. At 80% humidity, transmission was completely blocked. Temperature matters too: the virus spread more readily at 5°C (41°F) than at 20°C (68°F), and at 30°C (86°F) no transmission was detected at all.
This explains why flu season aligns so closely with the months when people spend the most time in dry, heated buildings. It also explains why warmer climates experience different patterns entirely.
Flu Season in the Southern Hemisphere
Because winter arrives at the opposite time of year below the equator, the Southern Hemisphere flu season typically runs from April through September, sometimes extending into October or November. Countries like Australia, Argentina, and South Africa see their peak flu activity during these months.
This staggered timing is useful for the Northern Hemisphere. Public health agencies watch how severe the Southern Hemisphere’s season turns out, since it can offer clues about which virus strains are circulating and what the upcoming U.S. season might look like.
Tropical Regions Don’t Follow the Same Pattern
Near the equator, flu doesn’t stick to a neat winter window. Countries in the tropics and subtropics often detect influenza year-round, and some experience two distinct peaks annually. A CDC study covering 138 countries found that 37 had one clear annual peak, while 17 had two separate peaks each year.
Most countries in Central and South America and in South and Southeast Asia see their primary flu activity from April through June. India adds a secondary peak between October and December. Sub-Saharan African countries near the equator report flu activity throughout the entire year, with no single dominant season. If you’re traveling internationally, flu risk doesn’t necessarily follow the calendar you’re used to at home.
When to Get Vaccinated
The CDC recommends getting your flu shot in September or October, ideally by the end of October. This timing lines up protection with the months when you’re most likely to encounter the virus. Getting vaccinated too early can be a problem: vaccine-induced immunity decreases over time, so a July or August shot may leave you less protected by February or March, when flu can still be circulating heavily. Adults 65 and older and pregnant women in their first or second trimester are especially advised against getting vaccinated too early for this reason.
Children who need two doses (typically those under 9 getting the flu vaccine for the first time) should start as soon as vaccine becomes available, since the second dose must come at least four weeks after the first. For everyone else, the September-to-October window is the sweet spot: early enough to build immunity before peak season, late enough that protection holds through the tail end of winter.
How Long You’re Personally at Risk
Even though the season spans about six months, your individual risk window depends on where you live, how much time you spend in crowded indoor spaces, and whether you’ve been vaccinated. Flu circulates at low levels even in summer, so exposure is technically possible any time of year. But the practical risk concentrates in those core winter months.
If you catch the flu, most people are contagious starting about one day before symptoms appear and remain contagious for five to seven days after getting sick. The illness itself typically lasts one to two weeks, with the worst symptoms in the first three to five days. Knowing the season’s timeline helps you plan vaccination, but it’s worth remembering that flu can show up earlier or later than expected in any given year.