It's caused by the immune system making antibodies to the killed viruses in the vaccine that help a person fight off the flu.

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Salk's vaccine, developed in the 1950s, involved injecting a virus that was "killed, " while Sabin's vaccine -- which he worked on in the 1960s and which was administered orally -- contained a weakened version of polio.

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The viruses in flu shots are killed during the production of the vaccine, which means they cannot cause infection.

CNN: Flu shot myths addressed

All the component parts of the vaccine are inactivated, killed, so they can't cause the diseases they are protecting against.

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The flu vaccine is comprised of killed or inactivated virus (three most likely strains of flu virus based on predictions from the previous flu season).

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According to Forbes contributor Steven Salzberg, a whooping cough epidemic in California that has killed nine babies may be the result of the burgeoning anti-vaccine movement.

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