LONG-TERM EFFICACY OF CONTINUING HEPATITIS-B VACCINATION IN INFANCY IN 2 GAMBIAN VILLAGES

In 1984, all non-immune children under the age of 5 years in the Gambi an 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% CI 93.0-96.0) and 95.3% (91.0-97.5), respectively. Breakthrough infections in vacci nated 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 tw o 4-year periods revealed that in the second period there were fewer t han half the expected numbers of infections. Our findings suggest that in Keneba acid Manduar longterm vaccination is progressively decreasi ng HBV transmission by chronic carriers, since their infectivity dimin ishes with time.