Around 1 billion people worldwide catch the flu each year, according to the World Health Organization. In the United States alone, the CDC estimates between 9.4 million and 51 million illnesses per season, depending on how severe that year’s circulating strains turn out to be. The most recent completed season (2023–2024) landed near the middle of that range, with an estimated 40 million Americans getting sick.
Why the Numbers Vary So Much
The wide range in annual estimates isn’t a sign of bad data. It reflects genuine year-to-year swings driven by which flu strains dominate, how well the vaccine matches those strains, and how much immunity the population carries from previous seasons. A mild year like 2011–2012 can produce fewer than 10 million U.S. cases, while a harsh season can push past 50 million.
These figures are also estimates, not direct counts. Most people who get the flu never take a lab test, so the true number of infections is always higher than what hospitals and clinics report. The CDC builds its annual burden estimates by starting with laboratory-confirmed hospitalizations from a surveillance network, then adjusting upward to account for people who were never tested, people whose tests missed the virus, and the large number of people who got sick but never saw a doctor at all. A 2010 national survey measuring how many people with flu-like illness actually sought medical care helps calibrate that final step.
Global Scale
Of the roughly 1 billion annual infections worldwide, 3 to 5 million qualify as severe illness, meaning cases that require hospitalization or carry a high risk of death. Seasonal flu causes between 290,000 and 650,000 respiratory deaths globally each year. These deaths concentrate in low- and middle-income countries where access to antivirals and intensive care is limited, and in older adults everywhere.
Not Everyone Who Catches It Feels Sick
About 16% of people infected with the flu virus never develop noticeable symptoms. They can still spread the virus to others, which is one reason flu moves so efficiently through households, schools, and workplaces. The 1 billion global figure includes these silent infections. When you hear someone say “I never get the flu,” there’s a reasonable chance they’ve had it without knowing.
Who Gets Hit Hardest
Flu infections are spread across all age groups, but hospitalizations cluster heavily at the extremes of age. During the 2022–2023 season, adults 65 and older accounted for roughly half of all flu-related hospitalizations in the U.S. The 65-to-74 age group alone had an estimated 86,400 hospitalizations, while those 75 to 84 added another 72,400 and those 85 and older contributed 37,300. Children under 5 made up about 15,500 hospitalizations that season.
The total hospitalization estimate for that season was around 379,300. One season later, in 2023–2024, the CDC estimated 470,000 flu-related hospitalizations, illustrating how much the burden can shift from one year to the next. Annual deaths in the U.S. range from 6,300 in a mild year to 52,000 in a severe one.
When Flu Season Peaks
In the U.S., flu season runs through fall and winter, with most activity peaking between December and February. The virus circulates year-round at low levels, but cold, dry air and indoor crowding create the conditions for rapid spread during winter months. The exact peak month shifts from season to season. Some years, activity surges early in November; others don’t peak until March.
How Much the Vaccine Reduces Cases
The flu vaccine’s effectiveness changes every year because the virus mutates and the vaccine must be reformulated to match. Over the past two decades, the CDC’s measured vaccine effectiveness has ranged from as low as 10% (in 2004–2005, when the vaccine was a poor match) to as high as 60% (in 2010–2011). Most seasons fall in the 30% to 50% range. The 2023–2024 season came in at 44%, and preliminary data for 2024–2025 shows 56%.
Even in years when the vaccine is only 30% to 40% effective at preventing infection, it tends to reduce the severity of illness in people who do get sick. That translates into fewer hospitalizations and deaths than the raw case numbers alone would suggest. The 2023–2024 season estimates reflect this: 40 million illnesses but a fraction of those resulting in hospitalization or death.
The Economic Cost
Beyond the health toll, flu costs the U.S. more than $87 billion annually when you combine medical expenses with lost productivity from missed workdays. That figure includes everything from doctor visits and prescriptions to the economic drag of millions of workers staying home sick for a week or more each winter.