Most people catch the flu about once every five years on average, though the frequency varies widely by age. In any given year, the flu causes somewhere between 9 million and 51 million illnesses in the United States alone, with around a billion cases worldwide.
How Many People Get the Flu Each Year
The numbers swing dramatically from season to season. At the low end, roughly 9 million Americans get sick in a mild year. At the high end, the toll is far greater: the 2024-2025 flu season was classified as high severity, producing an estimated 51 million illnesses, 710,000 hospitalizations, and 45,000 deaths in the U.S. Globally, the World Health Organization estimates about a billion flu cases each year, with 3 to 5 million of those becoming severe illness and 290,000 to 650,000 resulting in death.
Those numbers only capture people who feel sick. A CDC household study conducted over six flu seasons (2017-2023) found that about 8 percent of people who tested positive for influenza had no symptoms at all. The true number of infections in any given year is higher than illness counts suggest.
How Often You’ll Get It Over a Lifetime
Researchers at Oxford’s Pandemic Sciences Institute reconstructed lifetime flu infection histories and found that reinfections happen roughly once every five years on average. That pace isn’t constant, though. Young children get infected more frequently, and the interval between infections stretches longer as people move into adulthood. Over a typical lifespan, that adds up to somewhere around 15 to 20 bouts of flu, though many of those may be mild enough that you wouldn’t distinguish them from a bad cold.
Children Get the Flu Far More Often
Age is the single biggest factor in how often someone catches the flu. Children under 5 are about three times more likely to get a medically attended flu infection compared to adults in their 20s. Kids aged 5 to 9 face roughly 2.6 times the risk. The numbers drop through adolescence and bottom out in middle adulthood, where people in their 40s have the lowest infection rates.
After 65, the risk climbs again to about 1.6 times that of young adults. This likely reflects the increasing prevalence of chronic health conditions and the gradual weakening of immune defenses that comes with aging. For older adults, the concern isn’t just frequency but severity: flu-related hospitalizations and deaths are disproportionately concentrated in this age group.
The 2024-2025 season illustrated the toll on children in particular. An estimated 790 flu-related pediatric deaths occurred among children under 18, a stark reminder that flu hits the youngest and oldest hardest.
Why You Can Get the Flu More Than Once a Year
Getting the flu once doesn’t protect you for the rest of the season. Two main types of influenza circulate each year, influenza A and influenza B, and each has multiple subtypes or lineages. Catching one strain gives you little to no protection against the others. Even within a single subtype, the virus mutates constantly. A slightly different version of the same strain can reinfect you weeks or months later.
The flu’s reproduction number (a measure of how easily it spreads) ranges from 0.9 to 2.1 for seasonal strains, meaning each infected person typically passes the virus to one or two others. That’s enough to fuel widespread outbreaks every winter but not so high that the entire population gets hit at once, which is why the annual infection rate stays in the range of roughly 3 to 15 percent of the population rather than sweeping through everyone.
People with weakened immune systems, whether from illness, stress, medications, or chronic conditions, face a higher chance of catching the flu back to back within the same season.
How Much the Vaccine Reduces Your Risk
Flu vaccination doesn’t eliminate your chances of getting infected, but it meaningfully lowers them. The degree of protection varies each year depending on how well the vaccine matches circulating strains. Interim estimates from the 2025-2026 season give a snapshot of what to expect.
For children and adolescents, vaccination reduced outpatient flu visits by 38 to 41 percent and hospitalizations by 41 percent. For adults, the protection was somewhat lower: a 22 to 34 percent reduction in outpatient visits and a 30 percent reduction in hospitalizations. Adults 65 and older saw a 30 to 41 percent reduction in outpatient visits and 31 percent fewer hospitalizations.
Protection also varies by strain. The vaccine performed better against influenza B, cutting outpatient visits by 45 to 71 percent in children and 63 percent in adults. Against influenza A (the more common and typically more severe type), effectiveness was in the 30 to 38 percent range across age groups. Even in years when the match is imperfect, vaccination consistently reduces hospitalizations and deaths. During the high-severity 2024-2025 season, CDC estimates that flu vaccination prevented 10 million illnesses, 180,000 hospitalizations, and 12,000 deaths.
What Makes Some Seasons Worse Than Others
The five-fold difference between mild and severe flu seasons (9 million versus 51 million U.S. illnesses) comes down to a few factors. Which strain dominates matters enormously: seasons driven by certain influenza A subtypes, particularly H3N2, tend to be more severe. How much the circulating virus has changed since the previous year affects how much residual immunity the population carries. And vaccine uptake plays a role, since higher vaccination rates blunt the peak of transmission even when the vaccine isn’t a perfect match.
The timing and duration of flu season also shift. Most activity occurs between October and May in the Northern Hemisphere, with a peak typically falling between December and February. But some seasons start early and burn out fast, while others simmer for months. The unpredictability is part of why the annual illness range is so wide.