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Influenza (Flu) - Protect Yourself and Your Loved Ones
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Flu Activity
Reports & Surveillance Methods in the United States

Current U.S. Weekly Flu Report Current U.S. Map International Influenza Surveillance

Weekly Surveillance Reports

Years
Report for the week ending
Previous Years United States Surveillance Data: 1997 - 1998 through 2001 - 2002 Seasons

Overview of Influenza Surveillance in the United States

Woman Looking Through Microscope

The Influenza Branch at CDC collects and reports information on Influenza activity in the United States each week from October through May. The U.S. Influenza surveillance system has four separate components that allow the Influenza Branch at CDC to:

  • Find out when and where Influenza is circulating
  • Determine what type of Influenza viruses are circulating
  • Detect changes in the Influenza viruses
  • Track Influenza-related illness
  • Measure the impact Influenza is having on deaths in the United States

The Four Components of Influenza Surveillance:

1. World Health Organization (WHO) and National Respiratory and Enteric Virus Surveillance System (NREVSS) Collaborating Laboratories

About 75 WHO and 50 NREVSS collaborating laboratories located throughout the United States report the total number of respiratory specimens tested and the number positive for Influenza types A and B each week. Some laboratories also report the Influenza A subtype (H1N1 or H3N2) of the viruses they have isolated and the ages of the persons from whom the specimens were collected. Some of the Influenza viruses collected by laboratories are sent to CDC for more testing.

2. U.S. Influenza Sentinel Providers Surveillance Network

Each week, approximately 900 healthcare providers around the country report the total number of patients seen and the number of those patients with Influenza-like illness (ILI) by age group. For this system, ILI is defined as fever (temperature of >100°F) plus either a cough or a sore throat.

The percentage of patient visits to sentinel providers for ILI reported each week is weighted on the basis of state population. This percentage is compared each week with the national baseline of 2.5%. The baseline is the mean percentage of visits for ILI during non-Influenza weeks for the 2000-01 to 2002-03 seasons plus 2 standard deviations. Due to wide variability in regional level data, it was not possible to calculate region-specific baselines and it is not appropriate to apply the national baseline to regional level data.

3. 122 Cities Mortality Reporting System

Each week, the vital statistics offices of 122 cities report the total number of death certificates filed and the number of those for which pneumonia or Influenza was listed as the underlying or as a contributing cause of death. The percentage of all deaths due to pneumonia and Influenza are compared with a baseline and epidemic threshold value calculated for each week.

4. State and Territorial Epidemiologists Reports

State health departments report the estimated level of Influenza activity in their states each week. States report Influenza activity as no activity, sporadic, local, regional, or widespread. These levels are defined as follows:

No Activity: No laboratory-confirmed cases of Influenza and no reported increase in the number of cases of ILI.

Sporadic: Small numbers of laboratory-confirmed Influenza cases or a single Influenza outbreak has been reported, but there is no increase in cases of ILI.

Local: Outbreaks of Influenza or increases in ILI cases and recent laboratory-confirmed Influenza in a single region of the state.

Regional: Outbreaks of Influenza or increases in ILI and recent laboratory confirmed Influenza in at least 2 but less than half the regions of the state.

Widespread: Outbreaks of Influenza or increases in ILI cases and recent laboratory-confirmed Influenza in at least half the regions of the state.

Together, the 4 Influenza surveillance components are designed to provide a national picture of Influenza activity. Pneumonia and Influenza mortality is reported on a national level only. Sentinel physician and laboratory data are reported on a national level and by Influenza surveillance region. The state and territorial epidemiologists' reports of Influenza activity are the only state-level information reported.

It is Important to Remember the Following About Influenza Surveillance in the United States:

  • All Influenza activity reporting by states and health-care providers is voluntary.
  • The reported information answers the questions of where, when, and what Influenza viruses are circulating. It can be used to determine if Influenza activity is increasing or decreasing, but cannot be used to ascertain how many people have become ill with Influenza during the Influenza season.
 

 

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