Haemophilus influenzae type b disease in an Amish population: studies of the effects of genetic factors, immunization, and rifampin prophylaxis on the course of an outbreak.

In 1982, an outbreak of Haemophilus influenzae type b disease occurred in a 379-member Amish community. In an attempt to control the outbreak after the occurrence of the second case of disease, we investigated the combination of (1) rifampin chemoprophylaxis of all carriers of H influenzae type b and their household contacts from 1 month to 5 years of age and (2) H influenzae type b polysaccharide vaccine immunoprophylaxis of all community members 12 months of age and older. Despite our intervention, two additional cases of bacteremic H influenzae type b disease occurred in the ensuing 5 months, one in a 22-month-old infant who had been immunized at 19 months of age and the other in a child who had not been immunized because she was younger than 12 months of age. The outbreak ended following rifampin prophylaxis of all community members younger than 15 years of age. All of the children with disease were genetically related to one another, and three of the four were inbred. However, analysis of their coancestry revealed that neither the average level of kinship nor the average inbreeding level of the affected children differed significantly from those of the other children in the community. Furthermore, none of the four children with disease shared a human leukocyte antigen haplotype. Our observations suggest that inbreeding was not a risk factor in this community.(ABSTRACT TRUNCATED AT 250 WORDS)