Impact of Haemophilus influenzae Type b Polysaccharide-Tetanus Protein Conjugate Vaccine on Responses to Concurrently Administered Diphtheria-Tetanus-Pertussis Vaccine

Objective. —To assess whether serum antibody responses to diphtheriatetanus-pertussis (DTP) vaccine were affected by coadministration of Haemophilus influenzae type b capsular polyribosylribitol phosphate polysaccharide-tetanus protein (PRP-T) conjugate vaccine when given to patients at 2, 4, and 6 months of age. Design. —Randomized, double-blind clinical trial. Setting. —Urban Santiago, Chile. Patients. —Healthy infants assembled from health centers. Two hundred seventy-eight (74%) of 375 eligible infants participated; 222, who complied with the complete protocol, constituted the primary group under analysis. Interventions. —One of three vaccine regimens was given to study participants at 2,4, and 6 months of age, either DTP mixed in the same syringe as PRP-T (group 1); DTP and PRP-T given at separate injection sites (group 2); or DTP without PRP-T (group 3). Primary Outcome Measures. —Titers of serum antidiphtheria toxoid, antitetanus toxoid, and pertussis agglutinin antibodies were measured in blood samples taken from patients 2 months after each dose. Results. —Serum antidiphtheria toxoid and antitetanus toxoid responses showed no important depressions in the patients receiving PRP-T. In contrast, geometric mean titers (GMTs) of pertussis agglutinins, expressed as reciprocal serum dilutions, after both the second and third doses (GMT 2 , GMT 3 ) were lowest in group 1 (GMT 2 = 89; GMT 3 = 1230), intermediate in group 2 (GMT 2 = 123; GMT 3 = 1995), and highest in group 3 (GMT 2 = 210; GMT 3 =3090; P Conclusions. —Concurrent administration of PRP-T vaccine with DTP vaccine, either in the same syringe or at different sites, interfered with antipertussis responses to a primary series of immunizations. Although the clinical significance of this antagonism is uncertain, these data underscore the caution required in decisions to add new vaccines to existing immunization regimens. ( JAMA . 1992;267:673-678)