• Influenza surveillance helps to 1) determine when influenza viruses are circulating, 2) identify circulating strains, 3) detect changes in the viruses, 4) monitor influenza-related illness in the United States, and 5) measure the impact of influenza on deaths in the United States.

• CDC utilizes four components for influenza surveillance:

 

• World Health Organization and National Respiratory and Enteric Virus Surveillance System Collaborating Laboratories. Approximately 75 World Health Organization collaborating virology laboratories and approximately 50 laboratories from the National Respiratory and Enteric Virus Surveillance System located throughout the United States report the total number of respiratory specimens tested and the number positive for influenza by type and subtype each week. A subset of the influenza viruses isolated is sent to CDC for antigenic characterization.

 

• 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 mentioned in anywhere on the certificate.

 

• State and Territorial Epidemiologists Reports. State health departments report the estimated level of influenza activity in their state each week. When activity occurs, it is reported as sporadic, regional, or widespread which are defined as follows: Sporadic — Influenza cases, either laboratory-confirmed or influenza-like illness (ILI), are reported, but reports of outbreaks in places such as schools, nursing homes, and other institutional settings have not been received. Regional — Outbreaks of either laboratory-confirmed influenza or ILI are occurring in geographic areas containing less than 50% of the state's population. A geographic area could be a city, county, or district. Widespread — Outbreaks of either laboratory-confirmed influenza or ILI are occurring in geographic areas representing more than 50% of the state's population.

 

• U.S. Influenza Sentinel Physicians Surveillance Network. Approximately 450 physicians around the country report each week the total number of patients seen and the number of those patients with influenza-like illness by age group. States are not required to report influenza activity. All reporting is completely voluntary.

 

Each year from October through May, CDC provides weekly updates on U.S. influenza activity. The information is posted online at http://www.cdc.gov/flu/weekly/

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