The clinical and immunologic response of Chilean infants to Haemophilus influenzae type b polysaccharide-tetanus protein conjugate vaccine coadministered in the same syringe with diphtheria-tetanus toxoids-pertussis vaccine at two, four and six months of age

The safety and immunogenicity of a vaccine against Haemophilus influenzae type b consisting of purified polyribosylribitolphosphate conjugated to tetanus toxoid (PRP-T) was evaluated in 278 Chilean infants who were randomly assigned to one of three treatment groups: Group A, PRP-T mixed with diphtheria-tetanus toxoids-pertussis (DTP) vaccine in a single syringe and given as a single inoculation in one arm and placebo in the other arm; Group B, PRP-T given in one arm and DTP in the other arm; Group C, DTP given in one arm and placebo in the other. Infants were immunized at 2, 4 and 6 months of age and examined daily for 4 days after each immunization; serum PRP antibodies were measured at baseline and 2 months after each dose. The only adverse systemic reaction attributable to PRP-T beyond that caused by DTP alone was a 7 to 20% increase in febrile responses in the first 24 hours after the first and second doses of vaccine; the fevers were largely low grade and not accompanied by increased irritability, diminished activity or loss of appetite, compared with the group who received DTP without PRP-T. After the first dose 72% of infants who received PRP-T combined with DTP and 67% who received it in a separate arm attained antibody concentrations ≥0.15 μg/ml. After two doses of PRP-T, 93 and 95%, respectively, had concentrations ≥0.15 μg/ml and after three doses 100% of infants who received PRP-T had such titers. The anti-PRP geometric mean titer (4.80 μg/ml) in infants who received PRP-T combined with DTP, while high, was significantly lower than the group who received PRP-T in a separate arm (11.32 μg/ml). After the third dose the proportion of infants who achieved high anti-PRP concentrations (≥1.0 μg/ml) was 89% in those who received PRP-T combined with DTP and 98% in those who received PRP-T in a separate arm (P < 0.05). The results of these studies are sufficiently encouraging to support further evaluation of the coad-ministration of PRP-T and DTP in a single inoculation, in view of the practical advantages of such combined immunization.