Effect of priming with diphtheria and tetanus toxoids combined with whole-cell pertussis vaccine on the safety and immunogenicity of a booster dose of an acellular pertussis vaccine containing a genetically inactivated pertussis toxin in fifteen-to-twenty-one-month-old children

Objective: To evaluate the safety and the immunogenicity of a booster dose of recombinant acellular pertussis vaccine combined with diphtheria and tetanus toxoids (DTaP, Blocine SpA) in 15- to 21-month-old children primed in infancy with either whole-cell diphtheria-tetanus-pertussis (DTwP) vaccine or DTaP vaccine. Design: Open-label second phase of a double-masked, controlled trail, with masked analysis of serum samples. Participants and setting: Three hundred fifty children, 15 to 21 months of age, who had been primed at 2, 4, and 6 months of age with either three doses of DTaP vaccine (n = 173) or DTwP vaccine (n = 177). The children were enrolled in eight vaccination centers in Italy. Interventions: All children received a booster dose of the DTaP vaccine and were examined for safety at 48 hours and at 7 days after vaccination. Serum samples for evaluation of immunogenicity were obtained from 196 (55%) of the 350 children. Main Outcome measures: IgG antibodies to pertussis toxin (Ptox), filamentous hemagglutinin, 69-kilodalton protein, and tetanus toxoid were measured by enzyme-linked immunosorbent assay. Pertussis toxin-neutralizing antibodies were measured by the Chinese hamster ovary cell toxin neutralization assay. Main results: Adverse reactions to DTaP were infrequent, and there was no difference in the incidence of local or systemic reactions in children given DTaP as a fourth dose in comparison with a first dose. One month after the DTaP booster vaccination, both groups had 6- to 40- fold increases in serum antibody concentrations to all antigens tested; the concentrations against the three pertussis antigens were higher in the DTaP- primed children (p <0.05). The antibody titers to diphtheria and tetanus toxoids were higher in the DTwP-primed group (p <0.05), but both groups had protective titers. The geometric mean ratio of anti-Ptox neutralizing antibody per unit of IgG anti-Ptox antibody was higher in the DTaP-primed group (p <0.001). Conclusions: There are quantitative and qualitative differences in booster responses to DTaP vaccine in young children, depending on whether they were given DTaP or DTwP as primary immunization. This DTaP vaccine is safe and highly immunogenic as a booster.