Comparison of five different vaccination schedules with Haemophilus influenzae type b tetanus toxoid conjugate vaccine

Objective: To compare seroresponses to five different vaccination schedules for Haemophilus influenzae type b (Hib)-tetanus toroid conjugate (PRP-T) vaccine in infants. Design: Four different two-dose schedules were compared, with doses given at 1 and 3 months, 2 and 4 months, 2 and 6 months, or 4 and 6 months of age. One group received three doses at 2, 4, and 6 months of age. The PRP-T vaccine was given in the same syringe with diphtheria-tetanus-pertussis (DTP) vaccine; inactivated polio vaccine (IPV) was given in a separate syringe. Anti-Hib polysaccharide antibodies were measured by radioimmunoassay in sera taken before each immunization, 1 month after the second dose, at 7 and at 12 to 24 months of age. Subjects: A total of 196 healthy infants were enrolled between November 1990 and November 1992. Results: After one dose of PRP-T there were no significant differences in geometric mean antibody concentrations (0.09 to 0.13 mu g/ml) or in fold responses among the schedules. The response to the second dose was significantly higher than the response to the first dose given at the same age. The geometric mean antibody concentration was lower in the group vaccinated at 1 and 3 months than in the groups vaccinated at 2 and 4 months, 2 and 6 months, or 4 and 6 months. The three-dose schedule resulted in a significantly higher final antibody concentration than the best two-dose schedule (p < 0.001). In most children (64% to 93%), the antibody concentration remained at least 0.15 mu g/ml up to the age of 12 to 24 months. Conclusions: The Hib conjugate vaccine, PRP-T, administered concomitantly with DTP vaccine and IPV, was immunogenic with schedules starting at 1 to 4 months of age. Two injections of PRP-T vaccine were immunogenic enough to maintain protection up to 12 to 18 months of age.