Immunogenicity of Haemophilus influenzae type b polysaccharide-Neisseria meningitidis outer membrane protein conjugate vaccine in 2- to 6-month-old infants.

Fifty infants, 2 to 6 months of age, were vaccinated with Haemophilus influenzae type b capsular polysaccharide covalently linked to an outer membrane protein from Neisseria meningitidis group B. Subjects were given two injections and were randomly assigned to receive the injections separated by 1 or 2 months. Each dose contained 15 micrograms of polysaccharide and 51 micrograms of protein, or approximately twice the amount of polysaccharide as used in our previous trial (Lancet 1986;2:299). Fevers of 38.0 degrees to 38.8 degrees C developed in three infants (6%) within 24 hours after vaccination, but there were no other notable reactions. Following one injection, the geometric mean antibody concentration increased from 0.13 micrograms/mL in preimmune serum to 1.50 micrograms/mL in serum obtained 1 to 2 months later (P less than .001). After a second injection, there was a further increase in serum antibody (geometric mean = 3.11 micrograms/mL, P less than .007). The geometric mean antibody concentration of the group reimmunized 2 months after the first injection was higher than that in the group reimmunized after 1 month (3.95 v 2.32 micrograms/mL, P = .05, by analysis of covariance with age as the covariant). These data confirm our previous preliminary observations on the safety and immunogenicity of this new conjugate vaccine in infants 2 to 6 months of age. The data suggest that a 2-month interval between the first and second injections results in higher levels of serum antibody than a 1-month interval.