COMPARATIVE IMMUNOGENICITY OF 4 HAEMOPHILUS-INFLUENZAE TYPE-B CONJUGATE VACCINES IN ALASKA NATIVE INFANTS

We compared the immunogenicity of the four available Haemophilus influenzae type b (Hib) conjugate vaccines in Alaska Native infants. Three of the vaccines, Hib oligosaccharide-CRM197 (HbOC), polyribosylribitol phosphate-diphtheria toxoid (PRP-D) and polyribosylribitol phosphate-tetanus toxoid (PRP-T), were given at 2, 4 and 6 months of age, and the PRP Neisseria meningitidis outer membrane protein (PRP-OMP) conjugate vaccine was given at 2 and 4 months of age. Enrollment was largely sequential by vaccine availability beginning with HbOC and ending with PRP-T, A total of 225 infants completed the full vaccination series. Groups of infants receiving the different vaccines did not differ significantly by sex, ethnicity, degree Alaska Native or age at vaccination. The only vaccine that induced a response with the first 2-month dose was PRP-OMP; 91% of infants had greater-than-or-equal-to 0.15 mug/ml and 57% had greater-than-or-equal-to 1.0 mug/ml of anti-PRP antibody by 4 months of age. After two doses it also remained the most immunogenic. After the full three vaccine series, HbOC produced the highest concentrations of antibody; 94% of infants had greater-than-or-equal-to 1.0 mug/ml. PRP-T had approximately 75% of infants above this antibody concentration, and PRP-D had only 45% of infants with this level. Similar-rates of antibody decay were observed for all vaccine groups between 7 and 15 to 18 months of age and duration of antibody was primarily determined by peak antibody achieved. Differences in the pattern and degree of antibody response to each Hib conjugate vaccine were substantial.