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An estimated 200000 to 300000 hepatitis B virus infections occur annually in the United States. With acute infection, symptoms develop in fewer than 5% of infants, 5 to 15% of children between the ages of 1 and 5 years and 33 to 50% of older children and adults. However, the risk of chronic infection after acute infection is inversely proportional to age. The risk of chronic infection is highest for infants who acquire infection during the perinatal period (70 to 90%), lower for children younger than 5 years (20 to 50%) and lowest for older children and adults (5 to 10%). Therefore although only approximately 8% of acute infections in the United States occur in children younger than 10 years, these infections account for 20 to 30% of all chronic infections. Children usually acquire infection from infected mothers at the time of birth or from infected household contacts. The risk of hepatitis B virus transmission between children in day-care centers and schools is very low. Among adults and adolescents sexual activity and injecting drug use are the most common risks for acquisition of infection, yet at least 30% of reported hepatitis B among adults cannot be associated with an identifiable risk factor. Because chronic hepatitis B virus infection is associated with long term consequences of cirrhosis and primary hepatocellular carcinoma, prevention of chronic infection is the most important reason for vaccination against hepatitis B. Routine infant immunization is the most feasible, cost-effective means to control hepatitis B virus transmission.