Safety and immunogenicity of a new Haemophilus influenzae type b vaccine in infants under one year of age.

64 healthy infants 2-3 months old, were randomly assigned to one of three vaccination groups which received either diphtheria-pertussis-tetanus (DPT) vaccine, Haemophilus influenzae type b capsular polysaccharide polyribosyl-ribitol phosphate (PRP) vaccine, or PRP + P (with pertussis adjuvant) vaccine in three doses at intervals of 2 months. Local and systemic reactions occurred most frequently after DPT vaccination and least frequently for PRP alone. Data for 60 infants from whom complete sera sets were available indicated that 70% of the infants who received three doses of PRP + P showed 2-fold or greater increases in titres of antibody to PRP with final values above the level assumed to give protection against invasive H. influenzae type b disease (greater than or equal to 0.15 microgram/ml). In contrast, less than 10% of the infants who received PRP vaccine alone showed 2-fold or greater increases.