AIM:To observe the long-term effectiveness of low-dose immunization strategy and risk factors of HBsAg carriers in immunized children of Zhuang minorities of Longan County in the 9th year after infancy immunization.METHODS:Two epidemiologic methods,a cross-sectional follow-up study and a case-control study,were used for the evaluation of the serological effect and the determination of the risk factors. Hepatitis B virus markers were detected with radioimmunoassay.RESULTS:The protective anti-HBs-positive rate was 43.8% in 1183 children aged 1-9 years,who were immunized with three doses of 10&mgr;g hepatitis B vaccine in infancy according to 0, 1 and 6 months schedul.It declined from 87.9% in the first year to 37.1% in the 9th year after vaccination.The HbsAg-positive rate was 1.6%, not increasing with age during 9 years after the infant immunization program. Compared with 14.0% of HbsAg-positive rate of the baseline survey in 1985, the effectiveness of hepatitis B vaccine immunization was 88.6%. Of 36 immunized children with positive HBsAg, 89.1% were likely attributable to HBsAg positivity of their mothers.CONCLUSION:The long-term effectiveness of infancy low-dose hepatitis B vaccine immunization is high,and the booster is not needed 9 years after the vaccination in the Zhuang minority area where hepatitis B is highly endemic.A high-dose immunization strategy should be recommended in order to further decrease the current HbsAg-positive rate.