Randomized trial of the immunogenicity of fractional dose regimens of PRP-T Haemophilus influenzae type b conjugate vaccine

To assess the immunogenicity of more economical regimens of Haemophilus inf luenzae type b (Hib) conjugate vaccine, a randomized trial of fractional do ses of polyribosylribitol phosphate-tetanus toroid (PRP-T) Hib vaccine was undertaken in the Dominican Republic. Six hundred children were assigned to one of six regimens with PRP-T vaccine: full-dose, half-dose, and one-thir d-dose of Hib vaccine given separately or combined with diphtheria, tetanus , and pertussis (DTP) vaccine at ages 2, 4, and 6 months. Regimens that eli cited antibody levels > 1.0 mug/ mL in >70% of children and less than or eq ual to 0.15 mug/mL in > 90% of children were considered acceptable. At 1 mo nth post Dose 3, all regimens met the criteria for acceptable response. Amo ng those who received Hib as a separate injection, geometric mean concentra tions of anti-PRP bodies (GMCs) at age 1 month post Dose 3 were 11.2, 11.9, and 16.3 in the full, half, and one-third dose groups, respectively. Among those who received Hib and DTP combined, the GMCs were 6.4, 5.2, and 5.7 i n the full-, half-, and one-third-dose groups respectively.