Immunogenicity in infants of a vaccine composed of Haemophilus influenzae type b capsular polysaccharide mixed with DPT or conjugated to diphtheria toxoid

Fifty-three healthy infants received either Haemophilus influenzae type b capsular polysaccharide (PRP) mixed with diphtheria-pertussis-tetanus vaccine (DPT) or PRP conjugated covalently to diphtheria toxoid (PRP-D). The immunizations were given at 3, 5, 7, and either 14 or 18 months of age and were well tolerated. The geometric mean titers of antibody to PRP at eight months of age (after the first immunizations) were 0.26 micrograms/ml in the PRP + DPT group and 1.56 micrograms/ml in the PRP-D group. After the fourth dose, an IgG response was seen in both groups. The PRP + DPT group had a geometric mean level of antibody to PRP of 3.98 micrograms/ml at 19 months and the PRP-D group, 31.22 or 24.00 micrograms/ml at 15 or 19 months, respectively. Compared with previously published data of children immunized with one dose of PRP at 18 months, the mean level of antibody to PRP in the PRP-D group at 19 months was significantly (P less than .001) higher, whereas that in the PRP + DPT group was not.