Efficacy of Haemophilus influenzae type b vaccines in Massachusetts children 18 to 59 months of age

Since 1987 Haemophilus influenzae b (Hib) conjugate vaccines have been licensed for use in children ages 18 months and older. Before licensure there were no clinical trials of a single dose of any conjugate vaccine in children ages 18 months or older. To fulfill this need we performed an age- and residence-matched case-control study of the efficacy of Hib vaccines. In our study population the protective efficacy (PE) of Hib-diphtheria toxoid conjugate vaccine was 88% (95% confidence interval, 45 to 98%). No vaccine failures were observed with Hib oligosaccharide CRM197 diphtheria protein conjugate vaccine, but usage was not sufficient to establish efficacy: PE = 100% (95% confidence interval, -37 to 100%). The protective efficacy of Hib capsular polysaccharide vaccine was 18% (95% confidence interval -487 to 89%). We conclude that for children ages 18 to 60 months a single dose of the Hib conjugate vaccine, PRP-D, is protective against invasive Hib infections. Consistent with most studies Hib polysaccharide vaccine provided suboptimal protection.