Each year, an average of 5 to 10 percent of the U.S. population has symptomatic
influenza illness, 226,000 persons are hospitalized and 24,000 die due to
influenza-associated illness. Hospitalization rates are highest at the extremes of age, about one per 1,000 or higher in infants, persons age 65 and older and persons with chronic medical conditions. Ninety percent of deaths are in persons age 65 and older, but deaths also occur rarely in healthy children and young adults. Current
influenza vaccines are moderately effective, with current evidence suggesting that they can prevent about half of
influenza-associated symptomatic illness, outpatient visits, hospitalizations and deaths, with the evidence weaker for the most serious complications. Current licensed
vaccines have mild immediate adverse effects and serious adverse effects are rare. Annual estimates of
influenza vaccine effectiveness against the spectrum of clinical illness and in all age groups are needed to evaluate and support current
vaccine policies and to help guide more effective
vaccine development. Increased use of the current imperfect
vaccines could prevent substantial morbidity and mortality in the U.S.