In 1984, all non-immune children under the age of 5 years in the Gambian villages of Keneba and Manduar were vaccinated against hepatitis B virus (HBV). All children born in these villages since 1984 have been vaccinated in infancy. Despite a rapid fall in antibody concentrations, vaccine efficacy against HBV infection and chronic carriage of HBsAg has increased with time. Overall, vaccine efficacies in 1993 against HBV infection and chronic HBsAg carriage were 94.7% (95% Cl 93.0-96.0) and 95.3% (91.0-97.5), respectively. Breakthrough infections in vaccinated children largely originate from chronic HBsAg carriers. Thus, we tested 261 chronic carriers for HBV DNA and e antigen. The prevalence of these markers of infectivity, and the amount of HBV DNA, decreased greatly with age. Detailed studies of breakthrough infections over two 4-year periods revealed that in the second period there were fewer than half the expected numbers of infections. Our findings suggest that in Keneba and Manduar long-term vaccination is progressively decreasing HBV transmission by chronic carriers, since their infectivity diminishes with time.
PIP: Beginning in 1984, in the villages of Keneba and Manduar in The Gambia, health workers have continuously vaccinated all infants and all non-immune children less than 5 years old against hepatitis B virus (HBV). The children received 3 doses of 2 mcg plasma-derived HBV vaccine intradermally at 2-month intervals, 20 mcg vaccine intramuscularly followed by 2 intradermal doses of 2 mcg vaccine at 2-month intervals, or 3 doses of 20 mcg intramuscularly at 2-month intervals. Antibody titers of the first 3 groups of children in 1984 fell with time with no differences between the groups. Yet, vaccine efficacy against HBV infection and chronic carriage of hepatitis B surface antigen (HBsAg) increased with time. In 1993, overall vaccine efficacy against HBV infection stood at 94.7%. It was 95.3% for HBsAg carriage. Vaccine efficacy against HBV infection for children vaccinated between 1984 and 1989 was 89.8% compared to 97.7% for those vaccinated between 1989 and 1995. The vaccine efficacy against chronic HBsAg carriage was 95.1% and 97.1%, respectively. Breakthrough infections in children vaccinated during 1985-1989 and 1989-1993 were more frequent in children with low antibody concentrations (p 0.001). Among chronic HBsAg carriers, the prevalence of markers of HBV infectivity (i.e., HBVe antigen and HBV DNA) fell with age (p 0.01). During 1989-1993, the number of breakthrough infections was fewer than 50% of the expected number of infections. Since the infectivity of chronic HBsAg carriers is falling with time, the researchers submit that long-term HBV vaccination is reducing HBV transmission in Keneba and Manduar.