Seasonal Influenza, More Information
Questions & Answers
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- When is the flu season in the United States?
- How does CDC monitor the progress of the flu season?
- Why is there a week-long lag between the data and when it’s reported?
- How many people get sick or die from the flu every year?
- Is the “stomach flu” really the flu?
- Do other respiratory viruses circulate during the flu season?
In the United States, flu season occurs in the fall and winter. The peak of flu season has occurred anywhere from late November through March. The overall health impact (e.g., infections, hospitalizations, and deaths) of a flu season varies from year to year. CDC monitors circulating flu viruses and their related disease activity and provides influenza reports (called “ FluView”) each week from October through May. See Weekly U.S. Influenza Summary Update. Flu surveillance in the U.S. continues through the summer months with condensed reports available.
CDC collects data year-round and reports on influenza (flu) activity in the United States each week from October through May. The U.S. influenza surveillance system consists of five separate categories.
- Laboratory-based viral surveillance, which tracks the number and percentage of influenza-positive tests from laboratories across the country, and monitors for human infections with influenza A viruses that are different from currently circulating human influenza H1 and H3 viruses;
- Outpatient physician surveillance for influenza-like illness (ILI), which tracks the percentage of doctor visits for flu-like symptoms;
- Mortality surveillance as reported through the 122 Cities Mortality Reporting System, which tracks the percentage of deaths reported to be caused by pneumonia and influenza in 122 cities in the United States; and influenza-associated pediatric mortality as reported through the Nationally Notifiable Disease Surveillance System, which tracks the number of deaths in children with laboratory confirmed influenza infection;
- Hospitalization surveillance, which tracks laboratory confirmed influenza-associated hospitalizations in children and adults through the Influenza Hospitalization Network (FluSurv-NET) and Aggregate Hospitalization and Death Reporting Activity (AHDRA); and
- State and territorial epidemiologist reports of influenza activity, which indicates the number of states affected by flu and the degree to which they are affected.
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.
The influenza surveillance system is one of the largest and most timely surveillance systems at CDC. The system consists of 5 complementary surveillance categories. These categories include reports from more than 145 laboratories, about 3,000 outpatient health care providers, vital statistics offices in 122 cities, research and health care personnel at the Emerging Infections Program (EIP) sites, and influenza surveillance coordinators and state epidemiologists from all 50 state health departments and the New York City and District of Columbia health departments. Influenza surveillance data collection is based on a reporting week that starts on Sunday and ends on Saturday of each week. Each surveillance participant is requested to summarize weekly data and submit it to CDC by Tuesday afternoon of the following week. The data are then downloaded, compiled, and analyzed at CDC each Wednesday. The compiled data are interpreted and checked for anomalies which are resolved before the report is written and submitted for clearance at CDC. On Friday the report is approved, distributed, and posted on the Internet.
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 numbers vary, in the United States, millions of people are sickened, hundreds of thousands are hospitalized and thousands or tens of thousands of people die from flu every year. See “Burden of Influenza” for more information.
Many people use the term “stomach flu” to describe illnesses with nausea, vomiting or diarrhea. These symptoms can be caused by many different viruses, bacteria or even parasites. While vomiting, diarrhea, and being nauseous or “sick to your stomach” can sometimes be related to the flu — more commonly in children than adults — these problems are rarely the main symptoms of influenza. The flu is a respiratory disease and not a stomach or intestinal disease.
In addition to the flu virus, several other respiratory viruses also can circulate during the flu season and can cause symptoms and illness similar to those seen with flu infection. These non-flu 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.
- Page last reviewed: August 3, 2016
- Page last updated: May 4, 2016
- Content source:
- Centers for Disease Control and Prevention, National Center for Immunization and Respiratory Diseases (NCIRD)
- Page maintained by: Office of the Associate Director for Communication, Digital Media Branch, Division of Public Affairs