Standard and alternative regimens of Haemophilus influenzae type b conjugate vaccine (polyribosylribitol phosphate-tetanus toxoid conjugate vaccine) elicit comparable antibody avidities in infants

Background. Haemophilus influenzae type b conjugate vaccines are relatively expensive in the developing world. Previous study of the H. influenzae type b conjugate vaccine polyribosylribitol phosphate-tetanus toxoid conjugate vaccine showed that two dose and fractional three dose schedules elicit protective antibody concentrations equivalent to three full doses. Methods. Antibody avidity was measured in 73 of these vaccinees with a modified enzyme-linked immunosorbent assay using NH4SCN as the chaotrope. Avidity index (AI) is the molarity causing a 50% reduction in OD405. Results. The postprimary series AI was similar for all dosing regimens. Preboost AI was highest in those receiving three half-doses, although there was no statistical difference among groups. Rises in avidity from age 8 to 12 months were also similar among regimens. Our data support the equivalence of anti-polyribosylribitol phosphate IgG avidity in infants primed with these alternative regimens. Conclusions. Given the known correlation of avidity with assays of bacterial killing and memory priming, these potentially more economical alternative schedules should be studied in the developing world.