Haemophilus influenzae type b in a day care center: relationship of nasopharyngeal carriage to development of anticapsular antibody.

We measured anticapsular antibody to Haemophilus influenzae type b by radioimmunoassay in sera from 55 children attending a day care center in which two cases of H influenzae type b disease had occurred. The children ranged in age from 2 months to 36 months. Serum antibody levels in children attending the day care center were significantly higher than in age-matched controls (P less than .001), but in two different surveys one month apart 38% and 43% of the contacts had levels less than 100 ng/ml. Day care center children who were carriers of H influenzae type b had higher geometric mean antibody levels than noncarriers (478 ng/ml compared to 92 ng/ml, P less than .004). Nevertheless, six of 21 children (29%) with repeatedly positive cultures during four weeks of observation had concentrations of serum antibody less than 100 ng/ml when measured both 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.