While seasonal influenza (flu) viruses are detected year-round in the United States, flu viruses are most common during the fall and winter. The exact timing and duration of flu seasons varies, but influenza activity often begins to increase in October. Most of the time flu activity peaks between December and February, although significant activity can last as late as May.
The figure below shows peak flu activity in the United States by month for the 1982-1983 through 2019-2020 flu seasons. The “peak month of flu activity” is the month with the highest percentage of respiratory specimens testing positive for influenza virus infection during that influenza season. During this 38-year period, flu activity most often peaked in February (17 seasons), followed by December (7 seasons), January (6 seasons) and March (6 seasons).
In the United States, flu season occurs in the fall and winter. While influenza viruses spread year-round, most of the time flu activity peaks between December and February, but activity can last as late as May. The overall health impact (e.g., infections, hospitalizations, and deaths) of flu varies from season to season. CDC collects, compiles, and analyzes information on influenza activity year-round in the United States and produces FluView, a weekly surveillance report, and FluView Interactive, which allows for more in-depth exploration of influenza surveillance data. The Weekly U.S. Influenza Summary Update is updated weekly year-round.
The overall health impact (e.g., infections, hospitalizations, and deaths) from flu varies from season to season. CDC collects, compiles, and analyzes information on influenza activity year-round in the United States and produces FluView, a weekly surveillance report, and FluView Interactive, which allows for more in-depth exploration of influenza surveillance data. The Weekly U.S. Influenza Summary Update is updated each week. The U.S. influenza surveillance system is a collaborative effort between CDC and its many partners in state and local health departments, public health and clinical laboratories, vital statistics offices, health care providers, and clinics and emergency departments. Information in five categories is collected from eight different data sources that allow CDC to:
- Find out when and where influenza activity is occurring
- Track influenza-related illness
- Determine what influenza viruses are circulating
- Detect changes in influenza viruses
- Measure the impact influenza is having on hospitalizations and deaths in the United States
These surveillance components allow CDC to determine when and where influenza activity is occurring, determine what types of influenza viruses are circulating, detect changes in the influenza viruses collected and analyzed, track patterns of influenza-related illness, and measure the impact of influenza in the United States. All influenza activity reporting by states, laboratories, and health care providers is voluntary. For more information about CDC’s influenza surveillance activities, see the Overview of Influenza Surveillance in the United States.
Influenza surveillance data collection is based on a reporting week that starts on Sunday and ends on the following Saturday of each week. Each surveillance participant is requested to summarize the weekly data and submit it to CDC by the following Tuesday afternoon. The data are then downloaded, compiled, and analyzed at CDC. The data are used to update FluView and FluView Interactive on the following Friday.
In addition to flu viruses, several other respiratory viruses also circulate during flu season and can cause symptoms similar to those seen with flu infection. These respiratory viruses include rhinovirus (one cause of the “common cold”) and respiratory syncytial virus (RSV), which is the most common cause of severe respiratory illness in young children as well as a leading cause of death from respiratory illness in those aged 65 years and older.