The Haemophilus influenzae type b polysaccharide vaccine was licensed for use in the United States in April 1985. Postlicensure case-control efficacy studies have yielded markedly different estimates of efficacy, leading to contradictory recommendations to practicing physicians. To obtain additional information about the efficacy of the vaccine, we studied cases of invasive Haemophilus influenzae type b disease ascertained through active surveillance in areas with a total population of 34 million. We enrolled children 24 to 59 months of age who did not attend day-care centers. (Data from our day-care study have been published elsewhere.) For each case child, as many as three 24to 59-month-old control children were chosen from a roster of acquaintances supplied by the child’s parent. Conditional logistic regression was used, and vaccine efficacy was estimated to be 62% (95% confidence interval = 0%, 85%), which did not change significantly after adjusting for age and parental smoking, variables that were significantly different for case and control children. Results of this study support our previous finding of a positive protective efficacy, albeit lower than the efficacy of 90% found in children 18 to 71 months of age in the Finnish prelicensure trial. Pediatrics 1989;84:255-261; Haemophilus influenzae type b polysaccharide vaccine. ysaccharide vaccine (HBPV).’3 Although a large clinical trial in Finland in the middle 1970s demonstrated clinical protective efficacy of 90% (95% confidence interval = 55%, 98%) in children 18 to 71 months of age,4 postlicensure case-control efficacy studies, including one conducted by the Centers for Disease Control,58 yielded estimates of efficacy ranging from -69% to 88%. Interpretations of these data have resulted in contradictory recommendations to practicing physicians9 (PedComm: American Academy of Pediatrics Member Alert, November 13, 1987). Because of the developing controversy and our finding of a lower than expected efficacy in our first study,5 we initiated an additional, independent study to address the issue of HBPV efficacy in children 24 to 59 months of age. The study started in October 1986 and included patients from our active surveillance system who had a positive sterile-site culture result between January 1 and December 31, 1986 and who had not been eligible for our day-care-based HBPV efficacy
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