Decreased Haemophilus colonization in children vaccinated with Haemophilus influenzae type b conjugate vaccine.

OBJECTIVE: The incidence of invasive Haemophilus disease has unexpectedly decreased in unvaccinated children since the introduction of conjugate vaccine in the United States. The purpose of this study was to determine whether conjugate vaccination decreases colonization with Haemophilus influenzae type b. DESIGN: This study was a prospective, bimonthly survey of pharyngeal colonization with H. influenzae type b in children attending day care from October 1987 through September 1989. Vaccination status was determined from the medical record. SUBJECTS: We obtained 1188 pharyngeal cultures from 283 healthy children, 18 to 59 months of age; 51 children had received unconjugated polysaccharide vaccine (PRP), and 89 had received conjugate vaccine (94% PRP-diphtheria toxoid). MEASUREMENTS AND RESULTS: Multivariate analysis was used to test the possibility of an association between vaccination status and the rate of colonization with H. influenzae type b. Among children known to be exposed to at least one child with a positive culture result, the efficacy of conjugate vaccination to prevent H. influenzae type b colonization in an unmatched analysis was 64% (95% confidence interval: 5%, 86%; p = 0.02) and in a matched analysis 81% (95% confidence interval: 7%, 96%; p = 0.02). No effect on colonization was found with PRP vaccination. CONCLUSIONS: Conjugate vaccination decreases H. influenzae type b pharyngeal colonization and thereby may decrease transmission of this agent among healthy children.