Response to conjugate Haemophilus influenzae B vaccine among infants in Niamey, Niger

Despite near elimination of Haemophilus influenzae b (Hib) meningitis from several industrialized countries following introduction of conjugate Hib vaccines into infant immunization schedules, Hib remains a major cause of meningitis and pneumonia in resource-poor countries. In Niger, Hib causes nearly 200 cases of meningitis per 100,000 children < one year of age, and > 40% of cases are fatal. We evaluated the immunogenicity of Hib polysaccharide-tetanus toroid conjugate vaccine (PRP-T) administered in the same syringe as diphtheria-tetanus pertussis (DTP) vaccine among infants in Niger. Infants were randomized into group 1 (PRP-T at six, 10, and 14 weeks), group 2 (PRP-T at 10 and 14 weeks), or a control group (meningococcal A/C polysaccharide vaccine). By 14 weeks of age, all subjects in groups land 2 had greater than or equal to 0.15 mu g/ml of anti-PRP antibody, and 82% versus 76% had greater than or equal to 1.0 mu g/ml of antibody (P = not significant). By nine months of age the proportion of infants with greater than or equal to 0.15 and greater than or equal to 1.0 mu g/ml was group 1 = 97% and 76%; group 2 = 93% and 67%; controls = 10% and 2.6%. Four weeks after the first, second, and third doses of PRP-T, infants in group 1 showed geometric mean titers (GMTs) of 0.19, 3.97, and 6.09 mu g/ml while infants in group 2 had GMTs of 2.40 and 4.41 mu g/ml four weeks after the delayed first and second doses. Both PRP-T groups had significantly higher GMTs at 18 weeks and nine months of age than infants in the control group. The Hib PRP-T vaccine was immunogenic in infants In Niger. The strong response after PRP-T was initiated one month after the first DTP vaccination may reflect carrier priming. Two dose schedules of PRP-T should be given serious consideration, particularly if their reduced cost permits vaccine introduction that would be otherwise unaffordable.