RESUMO
BACKGROUND: Hepatitis B infection is hyper-endemic in Tonga and 19% of pregnant women test positive for hepatitis B surface antigen (HBsAg). Routine childhood immunization against hepatitis B was introduced in 1989 and the target for elimination was set at <1% HBsAg prevalence in children. A study conducted in 1998, a decade after the introduction of hepatitis B immunization, found the HBsAg prevalence to be 3.8% in pre-school children. The finding resulted in the strengthening of the delivery of hepatitis B vaccine with emphasis on providing the first dose within 24h after birth. The aim of this study was to measure the impact of improved immunization practices on the prevalence of hepatitis B infection in pre-school children, and to assess the progress towards hepatitis B elimination in Tonga. MEASURED OUTCOME: Prevalence of HBsAg antigen. TYPE OF STUDY: Cross-sectional study. METHODS: Children aged 6-59 months who were admitted to Vaiola Hospital, Nuku'alofa, Tonga, and had blood collected for clinical investigation, were tested for HBsAg with a rapid serological test (Abbott Determine). A total of 449 children were recruited and interviewed and 375 (84%) were tested for HBsAg. Convenience testing was chosen, as it was likely to be a relatively unbiased method in this situation where all children on the island have good and equitable access to hospital services. Immunization status was checked against the children's immunization cards and cross-checked against the records kept by the public health nurses. Information about socio-economic status, parent education, blood transfusion, breast-feeding, mode of delivery, and place of birth was collected through interviews with mothers using a standardized questionnaire. RESULTS: Three children tested positive for HBsAg resulting in a prevalence of 0.8% (CI 0.2-2.5%). Hepatitis B 1 (Hep B 1) immunization coverage was found to be high, 99.1% (CI97.7-99.7) and 91.9% (CI 88.9-94.2) of children received the first dose of hepatitis B vaccine within 24h after birth. Coverage for the third dose of hepatitis B vaccine was 97.6% (CI 95.5-98.7) and of the children with complete immunization 84.7% (CI 80.9-87.9) had received all three doses by 6 months of age. CONCLUSIONS: The results show that the instituted changes in the delivery of hepatitis B vaccine have been effective in reducing the transmission of hepatitis B to children and indicate that Tonga appears to have achieved the elimination target of less than 1% HBsAg prevalence in children. This convenience survey used a simple, cost effective and reproducible study design. Convenience testing can be recommended for surveillance of the effectiveness of immunization programmes in settings where the population has good access to health services.
