Haemophilus influenzae type b-specific antibody in infants after maternal immunization

To study the kinetics of Haemophilus influenzae type b (Hib)-specific antibody in infants born to mothers immunized with an Hib polysaccharide or one of two Hib conjugate vaccines.

Serum antibody to the polyribosylribitol (PRP) moiety of Hib was measured by radioimmunoassay and enzyme-linked immunosorbent assay at birth and at 2 and 6 months of age in infants born to women immunized with Hib polysaccharide or conjugate vaccine (PRP-D and HbOC). A subset of infants > or = 6 months of age was immunized with Hib conjugate vaccine after licensure of this vaccine for infants. A comparison group of 18 infants born to unimmunized women received the same Hib conjugate vaccine on a similar schedule.

Total PRP antibody concentrations were 1.50, 14.4 and 20.4 microg/ml in 2-month-old infants born to mothers immunized with polysaccharide, PRP-D and HbOC vaccines, respectively, and 2.54, 1.35 and 2.46 microg/ml in 6-month-old infants. Infants born to mothers immunized with polysaccharide vaccine had significantly less PRP antibody at 2 months of age but similar antibody concentrations at 6 months of age. Persistence or increases in total PRP antibody during 6 months were noted in 21 of 47 (44.6%) study infants. A subset of study and comparison infants was immunized with a mean of 2.6 doses of Hib vaccines between 6 months and 2 years of age, and all infants had total PRP antibody concentrations > or = 0.15 microg/ml.

Conjugate Hib vaccines administered during the last trimester of pregnancy resulted in significantly higher PRP antibody titers in infants at birth and 2 months of age than did polysaccharide vaccine. A subset of infants born to immunized mothers was subsequently immunized with Hib conjugate vaccine and had antibody concentrations similar to those in infants born to nonimmunized women.