Vaccination against hepatitis B and protection against chronic viral carriage in The Gambia. 358 children in the Gambian villages of Keneba and Manduar, where hepatitis B virus (HBV) infection is endemic, were vaccinated with plasma-derived vaccine against HBV according to one of four regimens and followed for up to 4 years. Two regimens by which vaccine was injected intradermally into children between 0 and 4 years old led to peak geometric mean (95% CI) concentrations of antibody against HBV surface antigen of 270 (202-358) and 555 (418-748) mlU/ml. The third regimen--intramuscular vaccination of children aged between 0 and 4 years--gave geometric mean peak antibody concentrations of 926 (765-1122) mlU/ml. A fourth regimen was intramuscular vaccination of children between 1 and 9 months old, which gave geometric mean antibody concentrations of 5431 (3903-75,456) mlU/ml. Despite these widely divergent responses and a 89% decay in antibody over the first 2 years, vaccination against HBV was 97% effective in preventing chronic infection. Vaccination was less effective in preventing uncomplicated infection: 5.3% of 264 vaccinees in Keneba and 19.1% of 94 vaccinees in Manduar tested positive for antibody to HBV core antigen. These "breakthrough infections" did not differ in frequency between regimens, and were associated with low initial antibody responses and chronic maternal carriage of HBV. 