Seasonal Influenza Vaccine Effectiveness, 2005-2017

CDC conducts studies to measure the benefits of seasonal flu vaccination each flu season to help determine how well flu vaccines are working. These vaccine effectiveness(https://www.cdc.gov/flu/about/qa/vaccineeffect.htm) (VE) studies regularly assess and confirm the value of flu vaccination as a public health intervention. Study results of vaccine effectiveness can vary based on study design, outcome(s) measured, population studied and the season in which the flu vaccine was studied.

CDC has been working with researchers at universities and hospitals since the 2003-2004 flu season to estimate how well flu vaccine works through observational studies using medically attended laboratory-confirmed flu as the outcome. This is the U.S. Flu Vaccine Effectiveness (VE) Network. The U.S. Flu VE Network currently consists of five study sites across the United States that measure the flu vaccine’s effectiveness at preventing outpatient medical visits due to laboratory-confirmed influenza. CDC’s observational studies at U.S. Flu VE Network sites measure outpatient visits* for laboratory-confirmed influenza infections using a highly accurate lab test called rRT-PCR to verify the outcome. These studies compare the odds of vaccination among outpatients with acute respiratory illness and laboratory-confirmed influenza infection to the odds of vaccination among outpatients with acute respiratory illness who test negative for influenza infection.

The overall, adjusted vaccine effectiveness estimates for influenza seasons from 2005-2016 are noted in the chart below. (Estimates are typically adjusted for study site, age, sex, underlying medical conditions, and days from illness onset to enrollment.)

*Estimate from Nov 2, 2015–April 15, 2016.
**As presented at June 21, 2017 ACIP meeting

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* From 2004-05 through 2010-11, the Network also enrolled inpatients.

† Vaccine effectiveness (VE) estimates for the 2008-2009 flu season have not yet been published.

‡ Number of patients used in VE calculation.