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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 this season?

CDC conducts studies each year to estimate how well the flu vaccine protects against having to go to the doctor because of flu illness. This season CDC is publishing information about how well the flu vaccine is working in the United States at two different times during the season: the middle and 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 composed of five dedicated sites across the United States, and 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's 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 current estimates of flu vaccine effectiveness this season?

CDC's mid-season VE estimates were published on February 20, 2014, in a Morbidity and Mortality Weekly Report entitled: "Interim Estimates of 2013-14 Seasonal Influenza Vaccine Effectiveness—United States." At the end of the season, CDC will provide a comprehensive estimate of VE that takes into account all of the data collected during the season. CDC's mid-season VE estimate was 61% for all age groups (95% confidence interval: 52% to 68%) against having to go to the doctor because of flu illness. This VE estimate means that getting a flu vaccine this season reduced the vaccinated population's risk of having to go to the doctor because of the flu by 60% for both children and adults.

Effectiveness against the flu A "2009 H1N1" virus, which is currently the most common flu virus spreading and causing illness in the United States this season, was 62% (95% CI: 53% to 69%) for children and adults. During the study period (Dec 2, 2013 – January 23, 2014), the 2009 H1N1 virus accounted for 98% of flu viruses detected. (Note: There were not enough influenza B or influenza A (H3N2) viruses detected during the study period to make a mid-season estimate of vaccine effectiveness against either of those viruses.)

(For background information on understanding VE estimates and confidence intervals, see Vaccine Effectiveness – How Well Does the Flu Vaccine Work? and go to the questions: "How does CDC present data on vaccine effectiveness" and "Why are confidence intervals important for understanding vaccine effectiveness?")

What do these VE estimates mean in terms of the benefits of vaccination this season?

Overall, these vaccine effectiveness estimates demonstrate the substantial public health benefit provided by the 2013-14 flu vaccine, particularly against the 2009 H1N1 virus, and indicate the vaccine is performing within the range expected. 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 60%, flu vaccination can reduce flu-related illness, antibiotic use, time lost from work, hospitalizations and deaths. These mid-season VE estimates indicate that at this time the 2013-14 flu vaccine is providing similar health benefits across all age groups. These findings are reassuring, but people should remain aware that despite vaccination, some people will still become infected with the flu this season. As a result, people at high risk of flu-related complications should not delay seeking medical care right away if they develop flu symptoms. In addition, clinicians should still suspect flu in vaccinated patients with acute respiratory illness (ARI).

How are flu vaccines working in people 65 and older this season?

CDC's mid-season VE estimates measured adjusted VE of 52% (95% CI: 2-76) among people 65 and older against flu A and B in the United States this season. This is slightly lower but generally similar to the vaccine benefits measured in other age groups this season. Adjusted VE in this age group against the 2009 H1N1 virus was 53% (95% CI: 2-78), suggesting the vaccine is providing protective benefit in the elderly against the predominant flu virus this season. This data is reassuring when compared to the substantially lower VE in the elderly measured last season (2012-2013) against then predominant H3N2 viruses. For more general information about VE in people 65 and older see, Vaccine Effectiveness – How Well Does the Flu Vaccine Work? and see the question "How effective is the flu vaccine in the elderly?"

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