Safety and Immunogenicity of Haemophilus influenzae Type b Polysaccharide-Diphtheria Toxoid Conjugate Vaccine (PRP-D) in Infants

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Abstract
Safety and immunogenicity of a Haemophilus influenzae type b polysaccharide-diphtheria toxoid conjugate vaccine (PRP-D) was evaluated in infants seven to 14 months of age. PRP-D (80% of subjects) or saline placebo (20%) was randomly and blindly administered (two doses separated by two months). Incidence of mild reactions lasting <48 hr did not differ significantly between the placebo and vaccine recipients. Preimmunization levels of antibody to PRP were ⩽0.15 μg/ml in 97% of subjects. A twofold increase in antibody concentration occurred in 88% of subjects following the first dose and in 99% following the second dose of vaccine. No change occurred in placebo recipients. Mean level of antibody and percentage of subjects with levels of antibody ⩾1 μg/ml after vaccination increased with increasing age. Responses were related to vaccine lot but not to sex, race, or geographic location. Two doses of PRP-D in infants seven months of age and older induced antibody levels equal to or greater than levels in infants 24 months of age given the polysaccharide alone.