MINIREVIEW Haemophilus influenzae Type a Infection and Its Prevention

infectionswere originally reported as “type b” or “non-type b.” Thenon-type b reports lacked epidemiological information such aspatients’ ages, predisposing illnesses, and mortality. A criticallevel of serum antibodies against Hib capsular polysaccharide,mostly immunoglobulin G, confers immunity to Hib by com-plement-mediated bacteriolysis (34, 35, 37). Routine immuni-zation with Hib conjugates in developed countries has led tosignificant reductions in the incidence of disease and carriageof this pathogen in the past decade (8, 9, 44). In the UnitedStates, following 10 years of Hib conjugate vaccine usage, theannual incidence of Hib meningitis among children less than 5years of age decreased from 10,000 to 200 cases (4).“Herd” immunity followed widespread usage of the Hib con-jugate vaccine as unvaccinated children benefited from thereduction in both the incidence of disease and the virtualelimination of carriage. The near-elimination of Hib disease insome populations led to the speculation that other