Antibody persistence after primary series of Haemophilus influenzae type b (Hib) vaccine and responses to boosters are seldom studied in Korean children. We performed this study to evaluate the antibody titer in relation to booster immunization of Hib vaccine in Korean children.

One hundred forty four children aged 12-23 months were enrolled in three university hospitals. The immunogenicity of boosters with Hib vaccine was assessed in children previously primed with Hib vaccine. Antibody persistence was also assessed in children who had received 3 doses of Hib vaccine without a booster. Anti-polyribosylribitol phosphate (PRP) IgG antibody levels and bactericidal titers were determined by enzyme immunoassay and bactericidal assay at the Center for Vaccine Evaluation and Study, Medical Research Institute, Ewha Womans University.

Prior to a booster in the second year of life, geometric mean antibody concentrations were 2.39 µg/mL and the percent of subjects who had a anti-PRP antibody level ≥1 µg/mL was 68.6%. After boosting, antibody concentration was 19.09 µg/mL and the percent of subjects who had a anti-PRP antibody level ≥1 µg/mL was 96.5%, which reflects previous immune priming. In subjects who had finished primary immunization only, the bactericidal titer was 3,946 and in subjects who had a booster, it was 11,205. Anti-PRP antibody level was correlated with serum bactericidal titer.

Many children aged 12-23 month old still had protective antibodies after recommended primary immunization only. A booster dose seemed to induce good anamnestic antibody responses in Korean children.