. Sixty serum samples, collected at age 7 months, with >2.0 g of anti-PRP IgG per ml were randomly selected for avidity determinations. Geometric mean IgG concentrations were 13, 14, and 17 g/ml for infants who received the full-dose (n ؍ 19), one-half-dose (n ؍ 19), and one-third-dose (n ؍ 22) regimens, respectively. SBA geometric mean titers (1/dilution) were 85.0, 82.0, and 76.1 in sera from infants receiving the full-, one-half-, and one-third-dose regimens, respectively. Avidity indices (mean ؎ standard error weighted average of NaSCN molar concentration ؋ serum dilution factor) were 71.9 ؎ 9.4, 123.6 ؎ 26.8, and 150.9 ؎ 24.9 for the full-, one-half-, and one-third-dose regimens, respectively. Upon comparison, the only significant difference (P ؍ 0.024) found was a greater avidity index for sera from infants receiving the one-third-dose regimen than for sera from infants receiving the the full-dose regimen. We conclude that fractional doses elicit similar functional antibody activities in infants with >2 g of anti-PRP IgG per ml, corresponding to 89, 90, and 97% of infants receiving three doses of either the full concentration or one-half or one-third of the labeled concentration, respectively. This approach offers an alternative strategy for the prevention of H. influenzae type b disease in countries with limited resources.
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