Immunogenicity of Haemophilus influenzae type b polysaccharide--diphtheria toxoid conjugate vaccine in adults.

The capsular polysaccharide of Haemophilus influenzae type b is a poor immunogen in human infants. In an attempt to enhance immunogenicity, this polysaccharide was covalently coupled to diphtheria toxoid and the conjugate tested as a vaccine in adult volunteers. Two injections of PRP-D vaccine were given, separated by one month. The anti-PRP antibody responses in this group were compared with those in a group receiving a comparable dose (20 micrograms) of conventional PRP vaccine. Both vaccines were well tolerated. A single injection of PRP-D was significantly more immunogenic than PRP, eliciting higher serum concentrations of total anti-PRP antibody 1 month later (geo means of 248 and 62 micrograms/ml, respectively; P less than 0.001). In addition, higher concentrations of IgG anti-PRP antibody were observed in the PRP-D group (P less than 0.001). One month after reinjection of vaccine, subjects receiving PRP-D showed a small but significant decline in total antibody (P = 0.03), whereas the serum antibody concentrations in the group that received PRP remained unchanged. At 12 months, the antibody concentrations of the two groups were not significantly different. Bactericidal activity and passive protection activity (infant rat model) were tested in pooled sera from the three highest and three lowest responders in each vaccine group; both PRP and PRP-D vaccines induced biologically active anti-PRP antibody. Thus PRP-D was found to elicit biologically active serum antibody and to be more immunogenic in adults than PRP vaccine; however, the duration of higher concentrations of antibody was transient.