Influenza, commonly known as the flu, is a respiratory illness affecting millions in the United States each year. Determining the exact number of flu cases is complex, as not everyone seeks medical attention or gets tested. Public health organizations use scientific estimation methods to understand the flu’s prevalence and impact, gauging the severity of each season.
Understanding Flu Case Data
Public health agencies, such as the Centers for Disease Control and Prevention (CDC), do not count every flu case. Instead, they rely on surveillance systems to generate estimates. These systems include networks like the Outpatient Influenza-like Illness Surveillance Network (ILINet) and the Influenza Hospitalization Surveillance Network (FluSurv-NET). ILINet gathers data from over 2,800 U.S. outpatient providers on patient visits for influenza-like illness (ILI).
FluSurv-NET collects data on laboratory-confirmed flu hospitalizations from acute care hospitals. These networks, along with virologic surveillance, help track the timing, intensity, and types of circulating flu viruses. Statistical modeling then uses this data to estimate the total number of illnesses, medical visits, hospitalizations, and deaths attributed to the flu each season.
Annual Flu Case Estimates
The estimated number of flu cases in the U.S. varies significantly each year. For the 2023-2024 flu season, preliminary estimates indicate approximately 40 million illnesses, higher than the 31 million estimated for 2022-2023.
The 2021-2022 season saw an estimated 11 million flu illnesses, considered low compared to pre-pandemic seasons. The 2019-2020 flu season had an estimated 35 million illnesses. Overall, between 2010 and 2024, the CDC estimates annual flu illnesses have ranged from 9.3 million to 41 million.
Influences on Flu Season Severity
Several factors contribute to the year-to-year fluctuations in flu case numbers and severity. The effectiveness of the annual flu vaccine and vaccination rates play a substantial role. When the vaccine closely matches circulating flu strains and more people are vaccinated, cases and their severity can decrease.
Dominant flu strains circulating each season also influence severity. Some strains, like H3N2, can lead to more severe illness. Collective immunity within the population, built from previous infections or vaccinations, affects how widely the virus spreads and the illness it causes. Public health measures, such as hand hygiene, mask-wearing, and social distancing, can also impact transmission rates.
Beyond Case Counts
The impact of influenza extends beyond illness counts, encompassing other public health metrics. Each year, the flu leads to many hospitalizations in the U.S. For the 2023-2024 season, estimates suggest around 470,000 flu-related hospitalizations. The 2022-2023 season saw an estimated 360,000, while the 2021-2022 season had about 120,000.
Flu-related deaths are another serious consequence. The 2023-2024 season had an estimated 28,000 deaths, the 2022-2023 season approximately 21,000, and the 2021-2022 season about 6,300. These figures highlight the burden flu places on the healthcare system and society. Influenza also carries an economic cost from lost productivity and healthcare expenditures, estimated at nearly $90 billion annually for the U.S. economy.