The most common flu vaccine is the inactivated (killed) influenza vaccine, which is prepared from the fluids of chick embryos inoculated with a specific type(s) of influenza virus. The strains of flu virus in the vaccine are inactivated with formaldehyde and preserved with thimerosal, which is a mercury derivative. (There is a limited supply of thimerosal-free or influenza vaccine with trace amounts of thimerosal and it is supplied in single dose vials, which do not require a preservative). This type of inactivated influenza vaccine is administered by injection into the muscle and contains three (trivalent) influenza virus strains – two type A and one type B strains. One flu vaccine, Afluria, is given by jet injector, a medical device which uses high pressure to administer the vaccine.
