Effectiveness of Haemophilus influenzae type b vaccines.
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PURPOSE: To determine the clinical effectiveness of Haemophilus influenzae type b (Hib) vaccines. STUDY IDENTIFICATION AND SELECTION: Computerized searches of MEDLINE, EMBASE and SCISEARCH databases were performed, and the reference list of each retrieved article was reviewed. Two prospective clinical trials of Hib polyribosyl ribitol phosphate conjugated with diphtheria toxoid (PRP-D) were identified. In addition, one cohort study of the PRP-D vaccine, two trials of the PRP vaccine, five case-control studies of the PRP vaccine and 10 randomized controlled trials of the immunogenicity of the PRP-D vaccine were identified. DATA EXTRACTION: Study quality was assessed and descriptive information concerning the study populations, the interventions and the outcome measurements was extracted. RESULTS: The difference in the effectiveness of the PRP-D vaccine between the prospective trials, in which a three-dose schedule had been used beginning at 2 to 3 months of age, was clinically important (37% v. 83%) but not statistically significant. The PRP vaccine, which induces lower antibody responses than the PRP-D vaccine does, was clinically effective only in a subgroup of one prospective trial; 90% effectiveness was reported among children 18 to 60 months of age. CONCLUSIONS: Hib vaccine appears to be less effective in high-risk populations. None the less, because of the large variation in baseline risk, the number of children who would have to be vaccinated to prevent one case of invasive Hib disease is substantially less for high-risk than for low-risk populations. The vaccination of children at high risk, such as native children, with the PRP-D vaccine using a four-dose schedule (at 2, 4, 6 and 14 months of age) seems warranted. The currently available evidence does not strongly support a policy of universal vaccination with either a one-dose or a four-dose schedule.