The changing epidemiology of invasive Haemophilus influenzae disease, especially in persons ≥65 years old

Background. Few studies have reported the epidemiological characteristics of Haemophilus influenzae disease among adults. Methods. Public health surveillance and hospital discharge data from Illinois were examined to determine the descriptive epidemiological characteristics and trends of invasive H. influenzae disease, and mortality data from Illinois were compared with data from several other states. Results. During January 1996-December 2004, 770 cases of invasive H. influenzae disease were reported to the Illinois Department of Public Health (Springfield). The incidence of disease increased from 0.4 to 1.0 cases per 100,000 persons, including an increase of incidence in adults aged >= 65 years from 1.1 to 3.9 cases per 100,000 persons. Nontypeable H. influenzae disease accounted for the greatest proportion of cases (35.8%-61.5%) in all but 1 age group. The number of cases of invasive nontypeable H. influenzae disease increased by 657%, from a low of 7 cases in 1996 to a high of 53 cases in 2004; as a proportion of annual cases, nontypeable H. influenzae disease increased from 17.5% in 1996 to 70.7% in 2004. Overall, the case-fatality rate was 12.7%, with the highest rate observed in persons aged >= 65 years (20.6%). The case-fatality rate was similar for the hospital discharge database and for Indiana, Maryland, Oregon, and Wisconsin (range, 12.9%-18.2%). Conclusions. In Illinois, the incidence of invasive H. influenzae disease increased from 1996 to 2004, and its epidemiological characteristics changed from a disease predominantly found in children and dominated by serotype b to a disease predominantly found in adults and dominated by nontypeable strains.