Several Haemophilus influenzae type b vaccines have been licensed and recommended for administration to children in the United States. These vaccines have consisted of purified polyribosylribitol-phosphate (PRP), the capsular polysaccharide of H influenzae type b,1 alone or covalently bound to one of several carrier proteins. Two of these saccharide-protein conjugate vaccines are now licensed, a polysaccharide-diphtheria toxoid conjugate (PRP-D)2 and an oligosaccharide-mutant diphtheria toxin conjugate (HbOC).3 Two others, a polysaccharide- Neisseria meningitidis outer membrane protein conjugate (PRP-OMPC)4 and a polysaccharide-tetanus toxoid conjugate (PRP-T),5 are currently in clinical trials.

One concern with the use of PRP vaccine was the suggestion that the incidence of invasive disease caused by H influenzae type b in the immediate period after immunization might be increased; this idea was supported by evidence from several sources. In a case-control study of the efficacy of PRP vaccine, Black et al6 found that 4 children were hospitalized for invasive disease within 1 week of immunization, a rate of invasive disease 6.4 times greater (95% confidence interval [CI], 2.1 to 19.2) than the background rate in unvaccinated children. In Minnesota, the relative risk for invasive disease in the first week after immunization was 6.2 (95% CI, 0.6 to 45.9),7 and the results of a study conducted by the Centers for Disease Control in six areas of the United States revealed a 1.8-fold (95% CI, 0.3 to 10.2) increase in the occurrence of invasive disease caused by H influenzae type b in the first week after immunization.8 Moreover, among 16 cases of disease caused by H influenzae type b occurring within 14 days of immunization that were passively reported to the FDA,9 10 were clustered within the first 72 hours.

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