Approximately every ten years, worldwide epidemics of influenza called pandemics occur. Thought to be caused by new strains of type-A virus, these pandemic viruses have spread rapidly, infecting millions of people.
Vaccines have been developed that have been found to be 70 to 90 percent effective for at least six months against either A or B types of the influenza virus, and a genetically engineered live-virus vaccine is under development. Currently, the United States Public Health Service recommends annual vaccination only for those at greatest risk of complications from influenza, including pregnant women and the elderly. Nevertheless, many other members of the general population request and receive flu shots every year, and even more are immunized during epidemic or pandemic cycles.