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The flu vaccine (administered as a shot or a nasal spray) is made of a combination of the different types of influenza viruses that research indicates will be most common during the upcoming flu season. For each type of influenza, there are numerous specific viruses that can be selected for use in the vaccine. The vaccine works by introducing small amounts of these influenza viruses to the body. Their presence stimulates the immune system to create antibodies against these specific viruses that help protect the individual from becoming infected with them. Each year, the specific viruses used in the vaccine will vary. Based on what you know of evolution, why might this be the case? Why can’t we reuse the same vaccine (and combination of viruses) from year to year?
