A major component in the prevention and control of influenza should be the use of killed influenza vaccines. These vaccines became possible after the first discovery of human strains of influenza virus in the 1930s. The ensuing decades have seen marked improvement in the available inactivated vaccines. Current vaccines have excellent reliability and assured potency, and they contain the proper antigens to match the frequent changes in circulating influenza viruses. Killed vaccines work by inducing serum antibodies against the hemagglutinin and neuraminidase of the vaccine strains, with sufficient antibodies ensuring protection against infection. The antibody responses to current vaccines appear to be adequate in all age groups. Although antibody responses are depressed in patients receiving chemotherapy or immunosuppressants, current vaccines do provide protection for most populations. Vaccines prevent the manifestations of disease by about 30 to 70 percent in all populations, and they reduce deaths in high-risk individuals by about 60 to 87 percent. Local adverse reactions to vaccine are quite common, but not severe. Fever, also somewhat common, usually does not last beyond 48 hours. Neurologic complications have not been observed since the use of the swine influenza vaccine of 1976. Killed vaccines should be given annually in the fall, but they can be given up to and during an outbreak. Target groups for vaccines have been defined by the Centers for Disease Control. In recent years, these groups have included physicians and nurses who give care to patients at risk for complications of influenza.