How does this medication work? What will it do for me? This medication belongs to a group of medications known as vaccines. It is used to prevent influenza (the flu). Influenza is a common viral illness caused by 2 types of virus: influenza A and influenza B. Each year, different strains (new, slightly different versions of the virus) appear. Scientists predict which strains will be most likely for the coming year, and then these strains are used to make up the year's influenza vaccine. Each year's influenza vaccine contains 3 to 4 virus strains that are likely to circulate in Canada in the coming winter. The vaccine only provides protection against the strains of flu virus used to prepare the vaccine. It usually takes 2 to 3 weeks for the protection against these viruses to be effective, and the protection lasts for 6 to 12 months. Annual vaccination is necessary to make sure you are covered for the new virus strains that are expected each year...