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Each year flu-causing viruses combine in ever-changing combinations to cause flu outbreaks around the world. Scientists and other influenza experts carefully monitor and study the types of flu viruses currently active to create the next flu vaccine.

The flu vaccine is made up of three different strains of (dead) flu virus. Because the flu viruses almost always change year to year, so does the vaccine. 

The 2011-2012 flu vaccine is made of the following:

  • A/California/7/2009 (H1N1)-like virus
  • A/Perth/16/2009 (H3N2)-like virus
  • B/Brisbane/60/2008-like virus

What happens if the scientists don't make a good match and the vaccine doesn't work?
Sometimes the strains that make up the vaccine are not a good match for the viruses that we encounter during flu season. In this case, you may get the flu even though you got a flu shot. The good news is your flu shot may still provide some protection by activating your immune system. Your symptoms may be less severe and you may not be sick as long.

Because there is no absolute way to know in advance that the viruses causing this year's flu are in the vaccine, it's that much more important to get a flu shot every year.