The composition of the vaccine is changed yearly to make certain the annual vaccine represents the strains that are most likely to be encountered during the upcoming season. A influenza is most commonly responsible for epidemics and type B influenza also causes significant illness.
Immunity lasts for one influenza season only.
Possible side effects are slight fever, fatigue, muscle ache, and tenderness and redness at the injection site. Current vaccines are more highly purified than older preparations and less likely to be associated with fevers and local reactions. The vaccine is derived from virus grown in embryonated hens eggs, and is contraindicated in individuals who are allergic to eggs. The virus in the vaccine has been killed by formalin and is not infectious. The influenza vaccine does not cause influenza or viremia.