Haemophilus influenzae type b disease. Incidence in a day-care center.

Haemophilus influenzae type b (HIB) disease was observed during a 14-month period in seven of 48 infants attending a day-care center. Surveillance studies showed that 28 (58%) infants had positive nasopharyngeal cultures for HIB; four infants were colonized with HIB for nine to 12 months. Ampicillin trihydrate prophylaxis failed to reduce the HIB carrier rate. Haemophilus influenzae type b polysaccharide vaccine was administered to 34 of the children. Sera obtained prior to immunization showed detectable antibody in all infants. Only nine (26%) infants had twofold or greater rises in serum HIB antibody titers after vaccination. Antibody response was independent of age, preimmunization antibody concentration, and HIB carrier status. In one infant, HIB meningitis developed four months after she received polysaccharide vaccine. This outbreak emphasizes that HIB is highly contagious in closed populations of young, susceptible infants.