Seasonal Influenza, More Information

When is the flu season in the United States?

In the United States, flu season occurs in the fall and winter. While influenza viruses circulate 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 a flu season 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 from October through May.

How does CDC monitor the progress of the flu season?

CDC collects data year-round and reports on influenza (flu) activity in the United States each week. The U.S. influenza surveillance system consists of five separate categories that allow CDC to:

• Find out when and where influenza activity is occurring
• Track influenza-related illness
• Determine what types of influenza viruses are circulating
• Detect changes in the influenza viruses collected and analyzed
• Measure the impact of influenza 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.

Why is there a week-long lag between the data and when it’s reported?

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.

How many people get sick or die from the flu every year?

Flu seasons vary in severity depending on a number of factors including the characteristics of circulating viruses, the timing of the season, how well the vaccine is protecting against influenza infection, and how many people got vaccinated. While the impact of influenza varies from season to season, it places a substantial burden on the health of people in the United States: millions of people become ill, hundreds of thousands are hospitalized and thousands or tens of thousands of people die from flu every year. See “Disease Burden of Influenza ” for more information.

Is the “stomach flu” really the flu?

Many people use the term “stomach flu” to describe illnesses with nausea, vomiting and/or diarrhea. These symptoms can be caused by many different viruses, bacteria or even parasites. While vomiting, diarrhea, and/or being nauseous or “sick to your stomach” can sometimes be related to the flu —these problems are rarely the main symptoms of influenza and are more commonly reported in children than adults. The flu is a primarily a respiratory disease, — common symptoms are fever (or feeling feverish/chills), cough, sore throat, runny or stuffy nose and/or muscle aches — and not a stomach or intestinal disease.

Do other respiratory viruses circulate during the flu season?

In addition to flu viruses, several other respiratory viruses also circulate during the flu season and can cause symptoms and illness 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.