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Variants within the "a" determinant of HBs gene in children and adolescents with and without hepatitis B vaccination as part of Thailand's Expanded Program on Immunization (EPI).

A total of 2229 children selected from five distinct areas of Thailand were screened for HBs antigen (HBsAg) by ELISA. Out of 51, forty-nine HBsAg-positive children were further examined for HBV-DNA by the polymerase chain reaction, utilizing the region of the hepatitis B Virus (HBV) genome encoding the major antigenic epitopes of hepatitis B surface antigen. Direct automated sequencing of the "a" determinant region revealed 11 of 49 children to display variable mutations. The vaccinated and nonvaccinated children had amino acid variants clustered between residues 120 and 160. Mutations between residues 120 and 160 were found at higher frequency in the vaccinated group (4/13; 30.8%) than in the nonvaccinated group (7/36; 19.4%), but this was not statistically significant. Infections with new HBV variants are contracted either vertically or horizontally within the group having received the vaccine, a finding confirmed by the presence of amino acid substitutions critical for immune escape. Hence, neither vaccine nor IgG has any apparent effect on those variants and the children turn into HBV carriers. However, the current vaccination program still efficiently protects perinatal transmission of HBV and unless long term studies lead us to conclude otherwise, inclusion of the variant strain(s) into a new vaccine formulation is not deemed necessary.