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.

Immunization of 2-month-old infants with protein-coupled oligosaccharides derived from the capsule of Haemophilus influenzae type b.

We studied an immunogen consisting of oligosaccharides derived from Haemophilus influenzae type b capsular polysaccharide (PRP) coupled to CRM197, a nontoxic relative of diphtheria toxin. Subcutaneous injections were given to eight subjects at ages 2, 4, and 6 months, simultaneously with conventional diphtheria-tetanus-pertussis (DTP) vaccine. After the first immunization, total serum anti-PRP antibodies declined in all subjects, but increased in most after the second immunization and after the third in seven of seven subjects analyzed. In these seven infants, the geometric mean level at age 9 months (0.73 micrograms/ml) exceeded by at least 40 times the means of historical control groups given DTP only or DTP plus (uncoupled) PRP vaccine. An isotype-specific assay showed that IgM antibodies increased after the first immunization with the coupled vaccine in all eight infants. Against the background of declining maternal IgG antibody, elevations in IgG antibody were detected after the second or third immunization in six of the eight. These six at age 9 to 11 months were immunized with (uncoupled) PRP vaccine, and a "boost" in anti-PRP antibody, including an IgG component, was found.