Immunity to hepatitis B in two birth cohorts given plasma-derived or yeast-derived vaccine

Aim. To determine the antibody response to either yeast-derived or low-dose , plasma-derived hepatitis B vaccine, in two cohorts of infants monitored b y an immunisation coordinator and immunised by general practitioners. Methods. Infants born to two cohorts of non-carrier mothers in Northland we re followed up, the first receiving a low-dose, plasma-derived vaccine, the second a yeast-derived vaccine. An immunisation coordinator enrolled the m others into the programme during pregnancy, promoted full immunisation agai nst hepatitis B and later obtained blood samples from their babies. In each cohort, four subsamples of babies, randomly assigned, were bled for estima tion of antibody levels to hepatitis B at ages 18, 30, 42 and 54 months (1 1/2, 2 1/2, 3 1/2, 4 1/2 years). No infant was bled more than once. Results. In both cohorts, antibody levels declined significantly with age. By age 4 1/2 years, 5.1% of children (95% confidence interval (CI): 3.5-7.1 ) immunised with yeast-derived vaccine were estimated to have antibody leve ls to hepatitis B below the acceptable level for protection of 10 IU/L. The proportion for those immunised with plasma-derived vaccine was 14.3% (95% CI: 7.4-24.1). Conclusions. Children receiving yeast-derived vaccine do not require a seco nd booster dose at school entry, although this might be considered at age 1 1. There are grounds to suggest that those who received low-dose, plasma-de rived vaccine (prior to 1990) should be offered a booster before age 11.