Priming and induction of Haemophilus influenzae type b capsular antibodies in early infancy by Dpo20, an oligosaccharide-protein conjugate vaccine.

A conjugate vaccine (Dpo20) was made by direct coupling of diphtheria toxoid and oligosaccharides obtained by periodate oxidation of Haemophilus influenzae type b capsular polysaccharide. This approach gave a higher multiplicity of saccharides per protein and greater immunogenicity in infancy than our previously studied conjugates. Thirty-three healthy infants received three sequential injections, and no serious side effects were observed. When Dpo20 was given with diphtheria-tetanus-pertussis vaccine at ages 2, 4, and 6 months, the geometric mean titer of anticapsular antibody rose to 5.9 micrograms/ml at age 7 months. Dpo20 given at 3, 5, and 7 months raised the mean to 3.2 micrograms/ml at 7 months (after two injections) and 15.4 micrograms/ml at 10 months. The antibodies included IgG and were bactericidal in vitro. Thus, antibody activities potentially protective against invasive H. influenzae b infections were induced in the most susceptible age range. The infants also became primed for mature-for-age responses to (unconjugated) polysaccharide vaccine given as a booster at age 12 months.