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Cold, Flu, & Cough Health Center

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Flu Shot Failure? Questions & Answers

The annual flu vaccine may not be as effective this year as in past years, the CDC has announced. Here's why.
WebMD Feature
Reviewed by Louise Chang, MD

The CDC reports this year's flu shot may not protect against a strain of influenza that's hitting the U.S.

Q. Does this mean the flu shot is useless?

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Not at all. Although this year's flu vaccine doesn't match two of the three main types of flu strains now in circulation, people who did get a flu shot and catch the flu get a much milder disease. This can make a life-or-death difference to people who are at high risk of flu complications, such as pregnant women, young children, the elderly, and those with chronic medical conditions such as diabetes, asthma, and heart disease.

At the Feb. 27 meeting of the CDC's Advisory Committee on Immunization Practices, University of Virginia professor James C. Turner, MD, said a UVA study found this year's vaccine to be 41% protective against serious flu illness. A similar study in 2003 found that year's vaccine to be 69% protective.

Turner, executive director for student health at UVA, said the school this year had a "robust" flu outbreak. Because UVA has a unique program that tracks students' flu vaccination status and uses rapid flu tests to detect flu outbreaks, CDC flu experts tell WebMD that this estimate is probably accurate, although the vaccine may be more or less protective in different regions of the country.

Q. Why was there a mismatch in this year's flu shot?

Flu vaccines contain a mixture of two influenza "A" virus strains -- this year they are types H3N2 and H1N1 -- and a "B" virus strain. These viruses mutate or "drift" a little each year, which is why the annual flu vaccine must be reformulated each year.

Most years, experts manage to match their flu vaccine selections in the spring to the strains that circulate later in the year throughout the U.S. and globally. In winter 2007, experts considered including a particular H3N2 strain of influenza A, but dropped the plan when they couldn't find samples that would grow properly in the manufacturing process.

Influenza A typically makes up about 85% of all flu cases. However, this year the particular H3N2 strain that was left out of the vaccine wound up being the dominant influenza A virus, which accounts for more than 60% of flu cases.

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