Progress Toward Elimination of Haemophilus influenzae type b Disease Among Infants and Children -- United States, 1987-1993

Haemophilus influenzae (Hi) causes disease among persons in all age groups, and Haemophilus influenzae type b (Hib) was the most common cause of bacterial meningitis among children in the United States. Since the introduction of Hib conjugate vaccines in 1988, the incidence of invasive Hib infections in the United States has declined among infants and children (1). Hib disease among children aged less than 5 years is now included in the list of vaccine-preventable diseases targeted for elimination in the United States by 1996 (2). Because Hi disease rates are generally higher for blacks than for whites, incidence rates are race-adjusted; race most likely reflects differing distributions of socioeconomic risk factors for Hi disease (e.g., household crowding) that may account for the variance in incidence rates. This report summarizes race-adjusted provisional data about trends in invasive Hi disease from two separate surveillance systems and emphasizes the need for early identification, investigation, and reporting of Hi cases. National Surveillance

State health agencies report weekly provisional notifiable disease data to the National Notifiable Diseases Surveillance System (NNDSS) through the National Electronic Telecommunications System for Surveillance (NETSS) managed by CDC's Epidemiology Program Office (3,4). Although invasive Hi disease did not become nationally notifiable until 1991, an increasing number of states voluntarily participated in weekly reporting to NNDSS during 1987- 1990. Because the primary purpose of NNDSS is timely surveillance of nationwide case information for many diseases, the information transmitted includes only basic demographic data on persons with invasive Hi disease. The capacity to electronically transmit supplemental information (e.g., the type of clinical illness, outcome, serotype causing disease, and Hib vaccination status) for cases of Hi disease is available through NETSS but is not widely used.

Among children aged less than 5 years, the race-adjusted incidence of Hi disease reported to NNDSS declined by 95%, from 41 cases per 100,000 in 1987 (seven states with 2.4 million children aged less than 5 years reported information) to two cases per 100,000 in 1993 (48 states with 18 million children aged less than 5 years reported information) (Figure_1). The incidence of Hi disease among persons aged greater than or equal to 5 years remained stable during this period (Figure_1). Laboratory-Based Surveillance

A laboratory-based system coordinated by CDC's National Center for Infectious Diseases included surveillance projects in four areas of the United States that participated from 1989 through 1993. The surveillance area population was 10.4 million in four states (three counties in the San Francisco Bay area, eight counties in metropolitan Atlanta, four counties in Tennessee, and the state of Oklahoma). Detailed information is routinely obtained on all cases of invasive Hi disease and includes serotype, clinical syndrome, outcome, vaccination status, and demographic information.

From 1989 through 1993, the race-adjusted incidence of Hib disease among children aged less than 5 years decreased rapidly when compared with the decrease in incidence of non-type b Hi disease among children (Figure_2). During the same period, this surveillance system indicated a rapid decline (93%) in the race-adjusted rates of all invasive Hi disease (including serotypes b and non-b) among children (from 41 cases per 100,000 to three cases per 100,000). Among children aged less than 5 years, the number of Hib cases declined by 98% (from 281 cases in 1989 to seven cases in 1993); the number of cases of non-type b Hi declined by 63% (from 52 cases in 1989 to 19 cases in 1993). Of the four Hib cases among children for whom Hib vaccination status has been determined, one had received the complete primary series. If projected to the U.S. population, an estimated 150 cases of Hib disease occurred among children aged less than 5 years in 1993. This system also indicated a substantial (89%) decrease in the number of cases of Hib disease among persons aged greater than or equal to 5 years (57 cases in 1989 compared with six in 1993).

Reported by: G Anderson, MPH, Bur of Disease Control, Oakland, California. L Smithee, MS, Oklahoma State Dept of Health. M Rados, MS, Dept of Preventive Medicine, Vanderbilt Medical Center, Nashville, Tennessee. W Boughman, MSPH, Veterans Administration Medical Svcs, Atlanta. National Center for Infectious Diseases; National Immunization Program; Epidemiology Program Office, CDC.

Editorial Note

Editorial Note: This report documents the continued decline in the incidence of all Hi and Hib disease in children aged less than 5 years in the United States. National surveillance monitors the occurrence of Hi disease which, in the past, was primarily caused by Hib organisms; the decline in incidence monitored by national surveillance most likely reflects a decline in Hib disease associated with use of Hib conjugate vaccines. In addition, the laboratory-based surveillance system provided direct evidence of a decline in Hib disease, which coincided with introduction and use of Hib conjugate vaccines for children aged 18 months in 1988 and infants aged greater than or equal to 2 months in 1990.

Based on findings from the National Health Interview Survey, in 1992, 67% of children aged 12-23 months had received at least one dose of Hib vaccine, and 36% had received three or more doses (CDC, unpublished data). Despite this incomplete level of vaccination coverage, surveillance indicates a decline of more than 90% in disease incidence, probably reflecting an unexpected additional benefit of conjugate vaccine use -- elimination of carriage (5), resulting in reduced exposure to the pathogen and decrease in disease incidence even among unvaccinated persons. The decrease in incidence of Hib disease among persons aged greater than or equal to 5 years in laboratory-based surveillance sites also is most likely a result of decreased carriage and transmission of the organism by infants and children. The availability of Hib conjugate vaccines, which are efficacious in children (6,7) and reduce carriage, make feasible the goal of elimination of Hib disease among children aged less than 5 years by 1996.

Achievement of the 1996 goal to eliminate Hib disease requires participation by all levels of the health-care provider system in collection of surveillance data (i.e., rapid identification, assessment, and prompt reporting of all cases) and optimal use of this information to prevent increased disease incidence among poorly vaccinated populations. To optimize surveillance, case reports should ideally satisfy four criteria. First, because Hib vaccines protect against serotype b organisms only, serotype should be determined and reported for all invasive Hi isolates. Second, to identify persons and groups at risk for Hib disease, vaccination status of all children with invasive Hib disease should be assessed. Third, to evaluate the possible role of incomplete or ineffective vaccination in persons with Hib disease, the date, vaccine manufacturer, and lot number for each Hib vaccination should be determined. Fourth, important measures of morbidity and mortality associated with Hi infections should be reported and include information on the type of clinical syndrome, specimen source (e.g., cerebrospinal fluid, blood, or joint fluid), and the outcome from disease. CDC is working with state health departments to optimize collection, compilation, and analysis of Hi surveillance data.

References

1. Adams WG, Deaver KA, Cochi SL, et al. Decline of childhood Haemophilus influenzae type b (Hib) disease in the Hib vaccine era. JAMA 1993;269:221-6.

2. CDC. Reported vaccine-preventable diseases -- United States, 1993, and the Childhood Immunization Initiative. MMWR 1994;43:57-60.

3. CDC. Mandatory reporting of infectious diseases by clinicians. MMWR 1990;39(no. RR-9).

4. CDC. National Electronic Telecommunications System for Surveillance -- United States, 1990-1991. MMWR 1991;40:502-3.

5. Takala AK, Eskola J, Leinonen M, et al. Reduction of oropharyngeal carriage of Haemophilus influenzae type b (Hib) in children immunized with an Hib conjugate vaccine. J Infect Dis 1991;164:982-6.

6. Black SB, Shinefield HR, Fireman B, et al. Efficacy in infancy of oligosaccharide conjugate Haemophilus influenzae type b (HbOC) vaccine in a United States population of 61,080 children. Pediatr Infect Dis J 1990;10:97-104.

7. Santosham M, Wolff M, Reid R, et al. The efficacy in Navajo infants of a conjugate vaccine consisting of Haemophilus influenzae type b polysaccharide and Neisseria meningitidis outer-membrane protein complex. N Engl J Med 1991;324:1767-72.

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