Objective: To evaluate antibody persistence one year after three-dose primary vaccination and booster immune response during the second year of life for a fully liquid diphtheria-tetanus-acellular pertussis-inactivated poliomyelitis-Haemophilus influenzae type b (DTaP-IPV-PRP∼T) vaccine. Methods: Infants at 18-19 months of age were given a booster dose of either DTaP-IPV-PRP∼T (group A) or DTaP-IPV plus PRP∼T at separate injection sites (group B), after primary vaccination at two, four and six months of age, with the same vaccines. Antibody concentrations were measured pre- and post-booster. Reactogenicity and safety were evaluated from parent reports. Results: Before the booster dose, 93.1% of group A and 95.1% of group B children still had anti-PRP antibody titers ≥0.15 μg/ml. All children had antibody levels believed to protect against tetanus, polio 1 (except one subject in group B), polio 2, polio 3, and diphtheria (except one subject in group A). At least 94% of children still had antibody concentrations ≥5 ELISA units (EU) to pertussis antigens (pertussis toxoid (PT), filamentous hemagglutinin (FHA), pertactin (PRN), fimbriae 2 and 3 (FIM2+3)). One month after the booster dose, all subjects achieved antibody concentrations or titers believed to be protective for PRP (polyribose ribitol phosphate)(≥1 μg/ml), diphtheria and tetanus (≥0.1 IU/ml) and poliovirus types 1, 2, and 3 (≥8 1/dil.), and at least 90.5% of subjects had four-fold increases in antibody concentrations to pertussis antigens following the booster. Anti-PRP geometric mean titers (GMTs) increased from 1.07 to 59.6 μg/ml and from 1.8 to 62.2 μg/ml in groups A and B, respectively. Both vaccine groups showed low reactogenicity rates. Conclusions: The fully liquid pentavalent DTaP-IPV-PRP∼T vaccine is highly immunogenic, with good antibody persistence for each antigen approximately one year after primary vaccination and strong booster responses at 18-19 months of age. Because this combined vaccine is fully liquid, requiring no reconstitution of lyophilized PRP∼T, the ease of use and proper administration are improved.
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