ATLANTA - Thanks to the widespread use of Haemophilus influenzae type b (Hib) polysaccharide vaccine over the last 13 years, the national incidence rate of invasive H. influenzae has decreased to 300 cases a year, according to a recent report from the Centers for Disease Control and Prevention (CDC).
Before the introduction of the vaccine, the majority of the nearly 12,000 cases reported per year were among children 5 years or younger, making it the most common cause of bacterial meningitis in that age group. Approximately 5% of the infected children died, and of those who survived the disease, 15% to 30% developed neurologic problems.
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Hib conjugate vaccines were initially introduced in 1989 for use in children 15 months and older and in 1990 were available for infants beginning at 2 months.
To continue the reduction in cases, the study investigators and the CDC recommend children be appropriately vaccinated to protect them and other children and infants in the community. Intensive efforts to increase vaccination levels among children in areas with coverage rates lower than 90% are needed to reach the elimination goal.
Invasive Hib disease among U.S. children 4 years and younger has become rare as a result of Hib conjugate vaccines. Disease incidence decreased 98% despite the fact that vaccination coverage of children 19 months to 35 months with three or more doses of Hib conjugate vaccines only reached 90% in 1995, according to the CDC.
During the prevaccine era, Hib invasive disease was the cause of more than 95% of H. influenzae invasive disease.
As with previous studies of invasive Hib disease in children 4 years or younger in the prevaccine era, higher H. influenzae and Hib incidence rates were noted in American Indian, African American and Hispanic children compared with Asian or Pacific Islander and white children.
The reduction in the number of reported cases is greater than anticipated, considering the efficacy of Hib conjugate vaccines and the coverage of a primary series. The investigators suspect this is likely due to the reduction of nasal pharyngeal carriage in vaccinated children, which indirectly protects unvaccinated children in the community.
The study results clearly indicate that "Hib invasive disease in children too young to have completed a primary series will continue to be a barrier for elimination unless Hib nasopharyngeal carriage rates in the population - including adults - reach zero."
The Advisory Committee on Immunization Practices (ACIP) recommends infants receive Hib at 2 months, 4 months and 6 months of age and a booster dose at 12 months to 15 months of age.
To document the number of reported H. influenzae invasive disease cases with onset in 1994 and 1995, the investigators used three sources of surveillance data: the National Notifiable Diseases Surveillance System, the National Bacterial Meningitis and Bacteremia Reporting System and laboratory-based surveillance sites.
During 1994, 10.5 million people were under active, laboratory-based surveillance for H. influenzae invasive disease. By 1995, 12.8 million were under surveillance.
During 1994, 1,277 cases of invasive H. influenzae cases were reported, followed by 1,332 cases reported in 1995. The overall incidence of invasive H. influenzae disease remained constant, but the incidence of invasive H. influenzae disease among children 4 years and younger was lower in 1995 than in 1994: 305 cases (1.56:100,000) compared with 364 cases (1.84:100,000), according to the CDC.
During the surveillance period, children 5 months and younger had the highest reported disease incidence of H. influenzae invasive disease, Hib invasive disease and also H. influenzainvasive disease of unknown serotype. The report suggested the high disease rate among this age group was because these children were too young to have completed the primary vaccination series. The lowest disease rates were among those 5 to 39 years.
During the prevaccine era, the highest incidence rate was among children 6 months to 17 months of age.
Only 36% of the 2,609 reported cases were serotyped, and a higher percentage (56%) of isolates were serotyped among children 4 years and younger compared with older cases (29%). Of the 376 serotyped isolates from children 4 years and younger, 184 were Hib. Hib represented 49% of invasive disease and 65% of meningitis.
The analysis included confirmed and probable cases. A confirmed case was defined as an illness clinically compatible with invasive disease with isolation of H. influenza from a normally sterile site. A probable case was defined as clinically compatible with detection of Hib antigen in cerebrospinal fluid.
Completion of a primary vaccination series was defined according to recommendations from the ACIP.
The average incidence of H. influenzae invasive disease from 1988 to 1990 among immunocompromised people and adults with specific underlying medical conditions was estimated at 1.7 cases per 100,000; 50% of cases examined in this study were attributed to Hib. None of the currently available Hib vaccines are licensed for use in adults.
Serotype information for all H. influenzae invasive disease cases is essential; therefore, monitoring the changing epidemiology of Hib invasive disease and looking for an increase or decline in incidence among children and adults is needed. This will generate data needed to examine new strategies for elimination.
Public health officials hope to have similar success with meningococcal and pneumococcal diseases using conjugate vaccine technology, said one of the study's authors, Bradley A. Perkins, MD, chief of the Meningitis and Special Pathogens Branch, CDC Division of Bacterial and Mycotic Diseases.
For more information:
- Bisgard KM, Kao A, Leake J, et al. Haemophilus influenzae invasive disease in the United States, 1994-1995: Near disappearance of a vaccine-preventable childhood disease. Emerging Infectious Diseases 1;2:1-9.
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