How do flu shots work?

They create memory: Flu-shots have parts of the virus that can't cause infection, but provoke an immune response. The virus continually changes, so that protection requires a new vaccine containing parts of the correct strain(s) of virus circulating that year, as predicted by the who. The immune reponse leads to "t-cells" that remember that part of the virus, and can quickly fight off live virus when you get exposed.

The immune system: Influenza vaccines (flu shots) work by stimulating the body's immune system to make antibodies to the flu virus. These antibodies are then effective at fighting off infections from "live" flu viruses. As a result, flu shots either greatly reduce the severity of seasonal flu infections or prevent people from developing an infection in the first place.