Epidemiology of Haemophilus influenzae type b disease and impact of Haemophilus influenzae type b conjugate vaccines in the United States and Canada.

Haemophilus influenzae type b (Hib) was the major cause of invasive bacterial disease in the United States and Canada before the introduction of Hib conjugate vaccines. Between 10000 and 20000 cases of Hib meningitis and other serious diseases occurred each year, leading to death in at least 3% of all patients and long term neurologic problems in up to 25% of survivors of meningitis. Introduction of Hib conjugate vaccines in Canada and the United States, first in children 18 months and older and later as a routine infant immunization, dramatically decreased the incidence of disease. By 1995 Hib disease levels had declined by more than 95% below preimmunization levels. The remarkably rapid reduction in disease incidence was partly because of the ability of the vaccine to reduce nasopharyngeal carriage of the organism, leading, when given widely, to reduced rates of exposure and infection even in those not immunized. Complete elimination of Hib disease in North America, however, will require achievement of relatively high coverage rates, especially in hard to reach populations where much of the remaining disease is occurring.