Haemophilus influenzae Type b in a Day Care Center: Relationship of Nasopharyngeal Carriage to Development of Anticapsular Antibody

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Abstract
Anti-capsular antibody [Ab] to H. influenzae type b was measured by radioimmunoassay in sera from 55 children attending a day care center in which 2 cases of H. influenzae type b disease occurred. The children ranged in age from 2-36 mo. Serum Ab levels in children attending the day care center were significantly higher than in age-matched controls (P < .001), but in 2 different surveys 1 mo. apart 38% and 43% of the contacts had levels < 100 ng/ml. Day care center children who were carriers of H. influenzae type b had higher geometric mean Ab levels than non-carriers (478 ng/ml compared to 92 ng/ml, P < .004). Of 21 children with repeatedly positive cultures during 4 wk of observation, 6 (29%) had concentrations of serum Ab < 100 ng/ml when measured by binding the 125I-derivative of the capsular antigen (polyribosyl-ribitol phosphate) or binding of 3H-polyribosyl-ribitol phosphate. These data may explain observations of H. influenzae type b disease in contacts of cases more than 30 days after hospitalization of the index patient.