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Cold, Flu, & Cough Health Center

2009-10 Influenza (Flu) Season

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    A season's severity is determined by comparing these measures with previous seasons.

    How effective is the seasonal flu vaccine?

    TThe effectiveness of flu vaccines can vary and depends in part on the match between the viruses in the vaccine and the flu viruses that are circulating in the community. If these are closely matched, vaccine effectiveness (VE) is higher. If they are not closely matched, VE can be reduced. During well-matched years, clinical trials have shown VE between 70% and 90% among healthy adults. For more information about seasonal flu vaccine effectiveness, visit "How Well Does the Seasonal Flu Vaccine Work?"

    Will this year's seasonal flu vaccine be a good match for circulating viruses?

    It's not possible to predict with certainty which seasonal flu viruses will predominate during a given season or what the severity, timing, or duration of a flu season will be. Flu viruses are constantly changing (called drift) – they can change from one season to the next or they can even change within the course of one flu season. Experts must pick which viruses to include in the vaccine many months in advance in order for vaccine to be produced and delivered on time. (For more information about the seasonal flu vaccine virus selection process visit "Selecting the Viruses in the Influenza (Flu) Vaccine.") Because of these factors, there is always the possibility of a less than optimal match between circulating flu viruses and the viruses in the seasonal flu vaccine. This season, it's likely that the 2009 H1N1 virus will circulate in the United States. A seasonal vaccine will not protect you against 2009 H1N1, but a vaccine against 2009 H1N1 is being produced.

    How are vaccine match and vaccine effectiveness determined?

    Over the course of a flu season CDC studies samples of flu viruses circulating during that season to evaluate how close a match there is between viruses in the vaccine and circulating viruses. In addition, CDC conducts vaccine effectiveness studies to determine how well the vaccine protects against illness.

    Can the seasonal flu vaccine provide protection against other seasonal flu viruses even if the vaccine is not a "good" match?

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