Vaccine antigen interactions after a combination diphtheria-tetanus toxoid-acellular pertussis/purified capsular polysaccharide of Haemophilus influenzae type b-tetanus toxoid vaccine in two-, four- and six-month-old infants

Objective. The safety and immunogenicity of a diphtheria-tetanus toxoid-acellular pertussis vaccine (DTaP; Trepedia(R))/Haemophilus influenzae b polysaccharide (PRP-T; ActHib(R)) combined vaccine (TriHibir(R); Pasteur Merieux Connaught) was compared with DTaP and PRP-T given at the same visit but at separate sites in a prospective multicenter, open label trial. Methods. Infants were randomized to four groups (three consistency lots of DTaP/PRP-T vs. one of the consistency lots given as separate vaccines); injections were administered at 2, 4 and 6 months of age. Pre-Dose 1 and post-Dose 3 sera were assayed for antibody titers against all antigens. Reactions to the vaccinations were assessed by parent questionnaire for 30 days after each injection visit. Results. Four hundred eighty-five infants were enrolled; 296 evaluable infants were included in the DTaP/PRP-T group compared with 70 infants in the DTaP and PRP-T vaccine group. Infants who received the combined vaccine had higher post-Dose 3 geometric mean antibody titers to diphtheria antitoxin (P < 0.01) and pertussis filamentous hemagglutinin (P < 0.05) and lower geometric mean antibody titers to tetanus antitoxin (P < 0.05) and Haemophilus influenzae b (Hib) polysaccharide (PRP) (P < 0.05). The geometric mean anti-PRP antibody titer in the DTaP/PRP-T group was 4.3 mu g/ml compared with 7.0 mu g/ml in the separate vaccine group (P < 0.05), and the percentage of infants with antibody titers greater than or equal to 0.15 and 1 mu g/ml were, respectively, 95 and 86%, whereas they were 100% for both titers in the separate vaccines group. DTaP/PRP-T vaccine given concomitantly or 1 month apart from hepatitis B vaccine and oral poliomyelitis vaccine caused no significant differences in immunogenicity or safety. The safety assessments for the DTaP/PRP-T vaccine showed no consistent differences in systemic or local injection site reactions compared with DTaP and PRP-T administered separately. Conclusion. Although the antibody responses to tetanus and Hib polysaccharide in the evaluated DTaP/PRP-T combined vaccine were significantly lower than those seen after separate DTaP and PRP-T administration, the combined vaccine elicited an immune response against diphtheria, tetanus, pertussis and Haemophilus influenzae b likely to confer protection.