An estimated 2 billion persons worldwide are infected with hepatitis B, and nearly 1 million die every year as a consequence of this condition. In 2004, the World Health Organization recommendation of universal infant immunization against hepatitis B had been implemented by 168 countries. Studies from hyperendemic countries indicate that immunization in infancy provides protection for at least 10 years, but little is known about the persistence of immunity in countries with a low prevalence of hepatitis B. Because the greatest risk of exposure in these countries occurs during adolescence and young adulthood, information about the need for booster immunization of teenagers is urgently needed. Zanetti and colleagues studied Italian children and Air Force recruits to determine the long-term effect of childhood immunization against hepatitis B.
The study began in 2003 and enrolled 1,212 children whose mothers were negative for hepatitis B surface antigen (HBsAg) and 446 recruits, representing all areas of Italy. All participants had been immunized before 1994. The average age of the children was 10.9 years; average age of the recruits was 21.8 years. Blood samples were obtained for antibodies to HBsAg (anti-HBs) and antibodies to hepatitis B core antigen (anti-HBc). Immunity was defined as anti-HBs concentrations of 10 mIU per mL (10 IU per L) or more. Individuals with lower concentrations were offered booster immunization, and their blood levels were reassessed after two weeks.
Protective levels of anti-HBs were detected in 779 (64 percent) of the children and 398 (89 percent) of the recruits. The mean antibody concentration was significantly higher in recruits. In neither group were factors such as sex, family income, and year of initial immunization associated with the probability of having protective antibody concentrations. Two weeks after receiving a booster dose, 332 (97 percent) of the initially nonprotected children had protective antibody levels. This amnesic response also was detected in 46 (96 percent) of the recruits following booster immunization. In both groups, the antibody levels were higher in individuals who had low but detectable levels than in those whose antibody levels were initially undetectable. The authors suggest that the higher levels of immunity in recruits could be a result of differences in the immunization doses used, the age at immunization, or the exposure to infection (e.g., through sexual activity).
The authors conclude that most individuals immunized in infancy retained protective levels of immunity more than 10 years after immunization for hepatitis B, and that a booster dose elicited a rapid and vigorous immunologic response in almost all of those with lower levels of immunity. In addition, they emphasize that immunologic memory for HBsAg may outlast the presence of detectable antibody. For these reasons, in addition to the decreasing incidence of hepatitis B in Italy, the authors conclude that routine booster immunization of teenagers who were immunized as infants is unnecessary. Additional research is needed to determine whether boosters are beneficial in later life.