Serum Anticapsular Antibody Response in the First Week after Immunization of Adults and Infants with the Haemophilus influenzae type b-Neisseria meningitidis Outer Membrane Protein Complex Conjugate Vaccine

Eight healthy adults and 48 infants 2 and 4 months old were immunized with Haemophilus injluenzae</it> type b-Neisseria meningitidis</it> outer membrane protein complex conjugate vaccine (PRP-OMP) to evaluate antibody kinetics in the first days after immunization. Five adults (63%) had some decrease in antibody, although the geometric mean did not decrease significantly. With one exception, the nadir occurred on postimmunization day 3. Seven had an antibody increase by day 7. Of the children, 6 (75%) of 8 and 17 (77%) of 23 had a decrease in antibody in serum obtained on day 2–3 after the first or second dose, respectively, the magnitude of which directly correlated with the preimmunization antibody concentration. However, the geometric mean did not decrease significantly. Within 1 week of immunization, 85% of infants had an increase in antibody, significantly greater after the second dose than after the first. A high concentration of maternally derived antibody before immunization correlated negatively with antibody response. Thus, a transient decrease in antibody occurs in most adults and infants 2–3 days after immunization with PRP-OMP followed by a prompt increase by day 7.