There has been uncertainty and controversy about the protective efficacy of Haemophilus influenzae type b polysaccharide vaccine almost since it first was licensed in the United States. This article will briefly review the available epidemiologic data about the protective efficacy of this vaccine in children with no recognized underlying illnesses.

H influenzae type b polysaccharide vaccine was licensed in the United States in April 1985, based on the results of a randomized clinical trial that was conducted in Finland.1 That study indicated that the vaccine's protective efficacy was 90% against invasive disease caused by H influenzae type b in children 18 to 71 months of age. Authorities recommended that all children receive the vaccine at 2 years of age and that it be administered to children up to the age of 60 months.2,3 The Immunization Practices Advisory Committee also recommended that children at increased risk (such as those who attend group day care) receive the vaccine at 18 months and again at 24 months of age because of its inconsistent immunogenicity when administered to 18-month-old children.2

Soon after its licensure, however, reports of vaccine failures began to appear. In some instances the vaccine failure could be attributed to an identifiable immune deficiency.4,5 However, Granoff et al6 reported 54 apparently normal children who had received the H influenzae type b polysaccharide vaccine but subsequently developed invasive disease caused by H influenzae type b. The majority of these children had normal serum concentrations of total immunoglobulins, IgG2, hemolytic complement, and antibody to tetanus toxoid (a T-cell-dependent antigen).

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