US Flu VE Network

The U.S. Flu Vaccine Effectiveness (VE) network is designed to provide estimates of clinical effectiveness of licensed vaccines by age group and by influenza type and subtype.

The network consists of seven study sites spread across the United States, including sites in the following states:

  • California;
  • Washington;
  • Wisconsin;
  • Michigan;
  • Tennessee;
  • Pennsylvania; and
  • Texas.

Enrollment in the annual study to measure VE begins after laboratory-confirmed influenza cases are reported in local surveillance for two consecutive weeks and continues for the rest of flu season.

The study uses a test-negative design which compares vaccination rates among persons with confirmed influenza illness versus persons with similar illness who do not have influenza based on laboratory tests. See Study Design Factors for more information.

Vaccination status is defined as receipt of at least one dose of the current season’s vaccine according to medical records, immunization registries, and/or self-report.

Patients are eligible if they are older than 6 months of age (thus being eligible to receive influenza vaccination) and they have reported acute respiratory illness with fever or new cough within the last 10 days.

Eligible patients complete an enrollment interview and answer basic questions regarding their influenza vaccination status, age, underlying health conditions, and other characteristics. Influenza vaccination status is later confirmed by reviewing records and immunization databases.

Following enrollment, a respiratory specimen is collected and tested for influenza virus using specific laboratory tests based on reverse transcription polymerase chain reaction (RT-PCR). Influenza viruses are then tested for influenza virus type A or B and subtype (i.e., H1N1 or H3N2).

Estimates are adjusted for study site, age, sex, self-rated general health status, race, Hispanic ethnicity, interval from onset to enrollment, and calendar time.

For more information about how CDC’s VE studies are conducted and how to interpret results, see Flu Vaccine Effectiveness: Questions and Answers for Health Professionals. See Seasonal Influenza Vaccine Effectiveness, 2004-2020 for a table that shows the overall adjusted VE and related reference for each season starting in 2004-05.