Infection with influenza viruses can cause severe morbidity and mortality among all age groups. Children, particularly those aged <5 years, have the highest incidence of infection during epidemic periods; however, the highest rates of influenza-associated hospitalizations and deaths are among the elderly (aged ≥65 years), children aged <2 years, and those of any age with underlying medical conditions. Each year, influenza-related complications are estimated to result in more than 226,000 hospitalizations. During 1976-2006, estimates of influenza-associated deaths in the United States ranged from approximately 3,000 to an estimated 49,000 persons. Annual vaccination is the most effective strategy for preventing influenza virus infection and its complications.