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Influenza - Seasonal - Myths and Facts
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  1. Fact #1: Even if I get the influenza vaccine, can I still be infected with influenza?
    Yes. Influenza viruses are always changing. They can change from the time the vaccine is recommended and the beginning of influenza season, or they can even change during a season. Each year, experts study thousands of influenza virus samples from around the world to figure out which viruses are making people sick and how these viruses are changing. With this information, they forecast which three virus strains are most likely to make the most people sick during the next influenza season. Each year, the seasonal influenza vaccine contains three influenza virus strains – one influenza A (H3N2) virus, one influenza A (H1N1) virus, and one influenza B virus. The selection of which viruses to include in the vaccine must be made in February of the prior year in order for vaccine to be produced in time for distribution the following season. When influenza viruses change, they may no longer closely match viruses used to make that season's influenza vaccine. This can make the vaccine less effective. But, even when this happens, the vaccine can still offer some cross-protection: The vaccine contains three viruses, so it can protect you against the other two viruses that may be making people sick. The immune protection you get from the vaccine can provide partial protection against influenza viruses that are related to those used to make the vaccine (this is called cross-protection). So while a less-than-ideal match can reduce vaccine benefit, the vaccine can still provide enough protection to make illness less severe and prevent influenza-related complications. A less-than-perfect vaccine is still the best protection we have against influenza. That is why CDC continues to recommend getting the vaccine even when there is a less-than-perfect match. Ref: http://www.cdc.gov/flu/about/qa/season.htm and http://www.cdc.gov/flu/about/qa/vaccineeffect.htm