Background. The development of a vaccine against Haemophilus influenzae type b (Hib) was stimulated by its recognition as a major pathogen of early childhood. The first vaccine to be developed was composed of the capsular polysaccharide of the organism, polyribosylribitol phosphate (PRP), and although effective in older children, it failed to protect those <2 years of age, the group with the highest burden of disease. The conjugation of PRP to protein led to a group of vaccines with enhanced immunogenicity and the ability to induce immunologic memory and thus the potential to protect in infancy.
Objectives. To review the trials of Hib conjugate vaccines in which protective efficacy in infants has been assessed and the experience in countries in which Hib conjugate vaccines have been introduced into the routine infant immunization schedule.
Discussion. Each of the Hib conjugate vaccines [PRP-diphtheria toxoid conjugate (PRP-D), PRP conjugated to outer membrane protein of Neisseria meningitidis group B (PRP-OMP), PRP oligosaccharides conjugated to mutant diphtheria toxin CRM197, (HbOC) and PRP conjugated to tetanus toxoid (PRP-T)] has been subjected to prospective clinical trials and all have demonstrated high protective efficacy with one exception: that of the least immunogenic vaccine, PRP-D, when used in a Native American population with a high level of natural disease. The trials have used different populations and different schedules, which limits conclusions about relative efficacies. However, it seems likely that all the vaccines are capable of high efficacy in populations with low levels and late age of Hib disease. Three vaccines (PRP-D, PRP-OMP, PRP-T) have been tested in populations with high rates of disease and only PRP-D has been found lacking. As predicted by immunogenicity data, PRP-OMP affords efficacy after one dose, and PRP-T is efficacious with an accelerated schedule. Of more practical significance the effectiveness of these vaccines when introduced into populations has been uniformly impressive.
Conclusions. Particularly where vaccine coverage is high, it is now likely that Hib disease can be eliminated using Hib conjugate vaccines in infancy.