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Q&A Flu Vaccine Effectiveness Estimates for 2013-14 Season

How does CDC determine how well the flu vaccine is working?

CDC conducts studies each year to estimate how well the flu vaccine protects against having to go to the doctor because of flu illness. For the 2013-14 season, CDC published information about how well the flu vaccine is working in the United States at two different times during the season: at the middle and the end of the flu season. CDC's estimates of the benefits of flu vaccine (also known as vaccine effectiveness or "VE" for short) are based on information CDC collects as the flu season progresses. Throughout the flu season, CDC collects data through the U.S. Influenza Vaccine Effectiveness (Flu VE) Network. The U.S. Flu VE Network is a collaboration between CDC and five institutions across the United States, where patients with flu-like symptoms are enrolled at doctor’s offices. CDC experts analyze data from these sites to determine how well the flu vaccine works in different age groups, and how well it works against the specific flu viruses that are spreading and causing illness. CDC's estimates of vaccine effectiveness can change over time as more information is collected.

Why does CDC publish estimates of flu vaccine effectiveness?

CDC publishes estimates of vaccine effectiveness to help inform prevention and treatment decisions made by doctors and other health care practitioners during the flu season.

What are CDC's estimates of flu vaccine effectiveness for the 2013-14 season?

Although CDC’s end of season VE estimates for the 2013-14 season have not been formally published at this time, they were presented to the Advisory Committee on Immunization Practices (ACIP) and are featured in a table of seasonal vaccine effectiveness on the CDC website. The predominant influenza virus during the 2013-14 season was the 2009 H1N1 virus. The 2013-14 influenza vaccine contained a virus similar to the 2009 H1N1 influenza virus. The 2013-14 season was the first season since 2009-2010 during which the 2009 H1N1 virus (which caused the 2009 influenza pandemic) was the predominantly circulating influenza virus. Results indicated that in 2013-14 vaccination provided moderate protection against influenza-associated medical visits caused by influenza. The overall VE for the 2013-14 season (adjusted for study site, age, sex, underlying medical conditions, and days from illness onset to study enrollment) was 51% across age groups (95% confidence interval: 43% to 58%). (For background information on understanding VE estimates and confidence intervals, see How does CDC present data on vaccine effectiveness? and Why are confidence intervals important for understanding vaccine effectiveness?) This VE estimate means that getting a flu vaccine during the 2013-14 season reduced the vaccinated population's risk of having to go to the doctor due to the flu by 51% for both children and adults.

What do these VE estimates mean in terms of the benefits of vaccination for the 2013-14 season?

Overall, these vaccine effectiveness estimates demonstrate the public health benefit provided by the 2013-14 flu vaccine, particularly against the 2009 H1N1 virus, and indicate the vaccine performed within the range expected. CDC's 2013-14 end of season VE estimates were published in December 2014 in a Morbidity and Mortality Weekly Report (MMWR) entitled: "CDC. Estimated Influenza Illnesses and Hospitalizations Averted by Vaccination—United States, 2013-14." Based on the model used in this study, influenza vaccination was estimated to have averted 7.2 million illnesses, 3.1 million doctor’s visits and 90,000 hospitalizations due to the flu during the 2013-14 season. Public health experts generally expect to see VE estimates of around 50-60% during flu seasons when most flu viruses spreading and causing illness are similar to the flu viruses the flu vaccine is designed to protect against. Even with moderate effectiveness of about 50-60%, flu vaccination can reduce flu-related illness, antibiotic use, time lost from work, hospitalizations and deaths.

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