Antibody levels achieved in infants by course of Haemophilus influenzae type B polysaccharide/diphtheria toxoid conjugate vaccine.

A Haemophilus influenzae type b (Hib) capsular polysaccharide (polyribosylribitol phosphate, PRP)/diphtheria toxoid (D) conjugate vaccine was given to 71 infants. The level of anti-PRP antibody believed to predict long-term protection, 1.0 microgram/ml, was reached in 50% of the infants who were vaccinated at 3, 5, and 7 months, and in 57% of the infants who received PRP-D at 7 and 9 months of age. A lower level of antibody, 0.15 micrograms/ml, which has been associated with protection at the time of assay, was reached in 92% and 87% of the two groups. Thus, the seroresponse was dependent upon both the frequency of immunisation and age of the infant. Comparison of these results with previous experience with PRP immunisation in Finland showed that PRP-D induced substantial antibody rises at an earlier age than did PRP.