Low genetic diversity of Haemophilus influenzae type b compared to nonencapsulated H. influenzae in a population in which H. influenzae is highly endemic. Infect. Immun

Immunization with Haemophilus influenzae type b (Hib) conjugate polysaccharide vaccines has dramatically reduced Hib disease worldwide. As in other populations, nasopharyngeal carriage of Hib declined markedly in Aboriginal infants following vaccination, although carriage has not been entirely eliminated. In this study, we describe the genetic characteristics and the carriage dynamics of longitudinal isolates of Hib, characterized by using several typing methods. In addition, carriage rates of nonencapsulated H. influenzae (NCHi) are high, and concurrent colonization with Hib and NCHi is common; we also observed NCHi isolates which were genetically similar to Hib. There is a continuing need to promote Hib immunization and monitor H. influenzae carriage in populations in which the organism is highly endemic, not least because of the possibility of genetic exchange between Hib and NCHi strains in such populations. Prior to the implementation of Haemophilus influenzae type b (Hib) vaccination, Hib was a major cause of invasive disease in the Northern Territory of Australia. Incidence rates of in-vasive H. influenzae disease in children under 5 years of age from mid-1985 to mid-1988 were 529/100,000 in the Aboriginal population and 92/100,000 in the non-Aboriginal population