Background
Every year, millions of people catch influenza, a viral disease of the airways. Most infected
individuals recover quickly, but elderly people, the very young, and chronically ill individuals
are at high risk of developing serious complications such as pneumonia; seasonal influenza
kills about half a million people annually. Small but frequent changes in the
influenza virus mean that an immune response produced one year by exposure to the
virus provides only partial protection against influenza the next year. Annual immunization with a vaccine that contains killed or live-attenuated (weakened) influenza viruses of the major circulating strains can reduce a person’s chance of catching influenza. Consequently,many countries run seasonal influenza vaccination programs that target elderly people and other people at high risk of influenza complications, and people who care for these individuals.