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Haemophilus influenzae type b (Hib) conjugate vaccines have successfully reduced the burden of invasive Hib disease in developed countries; however, their effectiveness in countries with a high incidence of pediatric HIV-1 is unknown.The effectiveness of Hib conjugate vaccine was prospectively evaluated in South African children. The burden of invasive Hib disease in children <1 year old was compared in 2 cohorts. The first cohort included 22 000 African children born in 1997 [969 (4.45%) of whom were estimated to be HIV-1-infected] who were not vaccinated with Hib conjugate vaccine. This group was compared with 19 267 children [1162 (6.03%) of whom were estimated to be HIV-1-infected] vaccinated at 6, 10 and 14 weeks of age with an Hib conjugate vaccine [TETRAMUNE (polyribosylribitol phosphate-CRM197-diphtheria-tetanus toxoids-whole cell pertussis)] between March, 1998, and June, 1999.The estimated burden of invasive Hib disease in nonimmunized HIV-1-infected children <1 year of age was 5.9-fold [95% confidence interval (95% CI), 2.7 to 12.6] higher than in HIV-1-uninfected children. The overall estimated effectiveness of Hib conjugate vaccine in fully vaccinated children <1 year of age was 83.2% (95% CI 60.3 to 92.9). Vaccine effectiveness was significantly reduced in HIV-1- infected [43.9% (95% CI −76.1 to 82.1)] compared with uninfected children [96.5% (95% CI 74.4 to 99.5);P < 10−5]. Among three of the fully vaccinated HIV-1-infected children who developed invasive Hib disease, the anti-Hib polyribosylribitol phosphate serum antibody concentrations were 0.23, 0.25 and 0.68 μg/ml.Although the Hib conjugate vaccine was less effective among HIV-1-infected than among uninfected children, it was 83% effective in preventing overall invasive Hib disease and therefore should be considered for inclusion in the routine vaccination schedule by other African countries.