Immunogenicity in infants of Haemophilus influenzae type B polysaccharide in a conjugate vaccine with Neisseria meningitidis outer-membrane protein.

63 children, aged 2-17 months, were given a new conjugate vaccine composed of the capsular polysaccharide of Haemophilus influenzae type b linked to a Neisseria meningitidis outer-membrane protein. Subjects under 7 months received two injections separated by 1 month, and older subjects received either one or two injections. There were no systemic reactions to this vaccine when it was given with aluminium hydroxide. A single injection of vaccine was highly immunogenic; the geometric mean serum anticapsular antibody concentrations before immunisation and 1 month later were 0.35 microgram/ml and 0.98 microgram/ml for babies of 2-3 months, 0.12 microgram/ml and 1.85 micrograms/ml for those of 4-6 months, and 0.15 microgram/ml and 4.1 micrograms/ml for those of 8-17 months (p less than or equal to 0.003 for each age group). After a second injection of vaccine, 80% and 76% of infants of 2-3 and 4-6 months, respectively, had antibody concentrations greater than 1.0 micrograms/ml. Most subjects showed evidence of IgG responses as measured by enzyme-linked immunosorbent assay. 6-12 months after immunisation, serum antibody levels had fallen (p less than 0.05) but they remained higher than those of unimmunized controls (p less than 0.001).