COMPARISON OF 5 DIFFERENT VACCINATION SCHEDULES WITH HAEMOPHILUS-INFLUENZAE TYPE-B TETANUS TOXOID CONJUGATE VACCINE

Objective: To compare seroresponses to five different vaccination sche dules 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 polysacc haride antibodies were measured by radioimmunoassay in sera taken befo re 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 enrol led between November 1990 and November 1992. Results: After one dose o f PRP-T there were no significant differences in geometric mean antibo dy concentrations (0.09 to 0.13 mu g/ml) or in fold responses among th e schedules. The response to the second dose was significantly higher than the response to the first dose given at the same age. The geometr ic 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 signi ficantly higher final antibody concentration than the best two-dose sc hedule (p < 0.001). In most children (64% to 93%), the antibody concen tration remained at least 0.15 mu g/ml up to the age of 12 to 24 month s. Conclusions: The Hib conjugate vaccine, PRP-T, administered concomi tantly with DTP vaccine and IPV, was immunogenic with schedules starti ng at 1 to 4 months of age. Two injections of PRP-T vaccine were immun ogenic enough to maintain protection up to 12 to 18 months of age.