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2009-10 Influenza (Flu) Season

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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?

Yes, antibodies made in response to vaccination with one flu virus can provide protection against related viruses. This can result in reduced vaccine effectiveness against the related viruses, but it can still provide enough protection to prevent or lessen illness severity and prevent flu-related complications. (However, a flu vaccine is not expected to offer cross-protection against viruses that are very different genetically from those in the vaccine.) In addition, it's important to remember that the seasonal flu vaccine contains three viruses so that even when there is a less than ideal match or lower effectiveness against one virus, the vaccine may protect against the other two viruses. For these reasons, even during seasons when there is a less than ideal match, CDC continues to recommend seasonal flu vaccination. This is particularly important for people at high risk for serious flu complications and for their close contacts.

Will the seasonal vaccine protect against the new H1N1 virus?

The seasonal vaccine is not expected to protect against the 2009 H1N1 virus because it is very different genetically from the seasonal H1N1 virus that is in this season's vaccine. That is why the government and manufacturers are producing a separate 2009 H1N1 vaccine designed to protect against the 2009 H1N1 virus.

What is CDC doing to monitor seasonal flu vaccine effectiveness for the 2009-10 season?

WebMD Public Information from the CDC

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