Before the development of an effective vaccine, Haemophilus influenzae type b was a common pediatric pathogen. The incidence of H. influenzae meningitis or invasive disease in children under the age of five years was one in 200 cases. Moreover, H. influenzae accounted for 70 percent of cases of bacterial meningitis in this age group. Since the vaccine's introduction in 1990, a growing body of data has suggested a dramatic decline in the incidence of H. influenzae meningitis in children. Schuchat and colleagues at the Centers for Disease Control and Prevention reported the results of laboratory-based surveillance for bacterial meningitis completed in 1995, five years after the vaccine was licensed.

Surveillance for invasive disease caused by H. influenzae, Neisseria meningitidis, group B streptococcus, Listeria monocytogenes and Streptococcus pneumoniae was performed in four states: Georgia, Tennessee, Maryland and California. The 22 counties studied represented approximately 10 million persons, or 3.9 percent of the United States population. Twenty-four percent of the study population was black. Invasive disease was defined as a disease in which a specific organism had been isolated from a sterile site, such as blood or cerebrospinal fluid. A case of invasive disease was considered to be meningitis if the patient's medical record listed meningitis as a diagnosis.

During 1995, bacterial meningitis was identified in 248 residents in the surveillance areas. The most common agent identified was S. pneumoniae (47 percent), followed by N. meningitidis (25 percent), group B streptococcus (12 percent), L. monocytogenes (8 percent) and H. influenzae (7 percent). The median age of the patients found to have meningitis was 25 years. The main pathogen causing meningitis in the neonatal patient group was group B streptococcus, and in those aged two to 18 years, N. meningitidis was the predominant pathogen. In patients over the age of 19 years, S. pneumoniae caused 62 percent of the reported cases of meningitis. Only one third of all cases of meningitis occurred in children under the age of five years. When invasive infections caused by H. influenzae in all age groups were totaled, 181 cases were reported, and meningitis was associated with 18 cases.

The authors conclude that the median age of patients with meningitis caused by one of these five pathogens has changed dramatically over a nine-year period. In 1986 the median age was 15 months, and by 1995 it was 25 years. In 1986, two thirds of patients with bacterial meningitis were between the ages of one month and five years. However, by 1995 meningitis had decreased by 87 percent in this age group. In addition, cases of meningitis in all age groups declined 55 percent.

Widespread use of the H. influenzae type b vaccine is primarily responsible for producing this major change in the epidemiology of bacterial meningitis. A formidable goal is to continue to develop effective vaccines for use in children, especially against group B streptococcus and pneumococcus, which are now the two predominant organisms causing invasive disease.