Prevention of Haemophilus influenzae type b colonization by vaccination: Correlation with serum anti-capsular IgG concentration

Concentrations of serum anti-Haemophilus influenzae type b (anti-Hib) capsu lar polysaccharide (CPS) greater than or equal to0.15 and greater than or e qual to1.0 mug/mL are widely used as surrogates for protection against inva sive Hib disease. However, the relationship between serum anti-Hib CPS foll owing immunization and protection against colonization is not known, making it difficult to evaluate new Hib vaccines or combination vaccines. In the Dominican Republic, nasopharyngealswabs were collected from 546 9-month-old infants who had received Hib conjugate vaccine at ages 2, 4, and 6 months and from 600 unvaccinated infants of the same age. The prevalence of Hib co lonization was lower among vaccinated infants than among unvaccinated infan ts (0.9% vs. 2,3%), Among vaccinated infants, protection against colonizati on was significantly correlated with anti-Hib CPS concentrations greater th an or equal to5 mug/mL 1 month following the third dose of vaccine. These r esults suggest that the concentration of serum anti-Hib CPS needed for prot ection against colonization is greater than that needed for protection for invasive disease.