Flu season in the United States runs through fall and winter, with activity typically picking up in October and peaking between December and February. It can remain elevated into May. The exact timing shifts from year to year, but this general window holds for most seasons.
When Flu Season Officially Starts and Ends
The CDC tracks influenza on a surveillance calendar that begins around the first week of October (epidemiologic week 40) and runs through late September of the following year. For the 2025–2026 season, that window is September 29, 2025, through September 26, 2026. But the surveillance calendar is just the reporting period. The actual season of elevated illness is shorter.
In most years, flu activity starts climbing in October, hits its highest point sometime between December and February, and tapers off by spring. Some seasons peak as early as December, others as late as March. The CDC considers the season to have officially started after consecutive weeks of elevated influenza activity appear across its surveillance systems, including rising rates of doctor visits for flu-like illness and increasing percentages of lab specimens testing positive for influenza.
For the current surveillance period, the national baseline for flu-like illness visits is 3.1%. When the percentage of outpatient visits for fever, cough, and sore throat exceeds that threshold for consecutive weeks, it signals the season is underway.
Which Strains Dominate and When
Two main types of influenza circulate each year: influenza A and influenza B. They don’t follow identical schedules. Influenza A tends to spread more aggressively and dominates early in the season. Influenza B typically appears later and spreads more commonly in schools and among children. This is why you can catch the flu twice in one season, once from each type, and why a late-winter illness that “feels like the flu” may genuinely be a second round.
Flu Season in the Southern Hemisphere
Because seasons are reversed below the equator, the Southern Hemisphere’s flu season runs roughly from April through September, sometimes stretching into October or November. Public health officials in the U.S. watch these patterns closely because what happens in countries like Australia and Argentina often previews which strains and severity levels will arrive in North America a few months later.
Tropical Regions Follow Different Rules
The neat fall-and-winter pattern only applies to temperate climates. Countries near the equator often detect influenza year-round, sometimes with multiple peaks in a single year. A CDC collaborative study found that most countries in Central and South America and in South and Southeast Asia experience a primary wave of influenza activity from April to June. India sees an additional peak between October and December. Sub-Saharan African countries near the equator circulate influenza throughout the year with no single dominant season.
This matters for travelers. If you’re heading to the tropics in July thinking flu season is over, it may not be.
How COVID Changed the Pattern
The COVID-19 pandemic disrupted normal flu circulation worldwide. In some regions, entire flu peaks vanished during 2020 and 2021 as masking, distancing, and travel restrictions suppressed transmission. When flu returned, it didn’t always follow its old schedule. Researchers documented unexpected off-season peaks, like a May–July influenza wave in Senegal in 2022 that fell outside the country’s normal timing. Evidence suggests that active circulation of SARS-CoV-2 may have suppressed influenza through a phenomenon called viral interference, where one widespread respiratory virus makes it harder for another to gain a foothold.
In the U.S., flu seasons since 2022 have largely returned to their pre-pandemic timing, but public health officials continue to monitor whether the traditional December-to-February peak holds or whether the post-pandemic landscape produces more variability.
Best Time to Get Vaccinated
September and October are the recommended months for most people to get a flu shot. Vaccination earlier than that, in July or August, is generally not advised because vaccine-induced immunity can wane over the course of the season. This is especially relevant for older adults, who tend to lose protection faster. Since flu activity usually doesn’t peak until December at the earliest, a September or October vaccination lines up your strongest immunity with the weeks when you’re most likely to be exposed.
If you miss that window, getting vaccinated later in the fall or even into winter still offers protection. Flu can circulate well into spring, so a January vaccination isn’t wasted.