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The Special Hib Threat

Over the past few years, there have been about 2,400 to 2,900 cases of meningococcal infection reported annually to the national Centers for Disease Control in Atlanta. Some 46 to 47 percent are in children and teens (who compose 27 percent of the population). And about a quarter of all the cases are in preschool-age children (who represent about 6 percent of the population). The overall incidence is 1 to 1.5 cases per 100,000 population.

While there are no comparable figures for Haemophilus influenzae type b (Hib) infection, CDC has studied comparative incidence in the specific population subgroups of black non-Hispanics, white non-Hispanics, and Hispanics, reports medical epidemiologist Janet Mohle-Boetani, M.D., of CDC's Meningitis and Special Pathogens Branch. These figures consistently show that Hib cases outnumber meningococcal meningitis cases by about 3 to 1.

Among small children, the comparative attack rate has been far higher. According to CDC, before the introduction of the first vaccine, 1 in 200 children in the United States developed an invasive Hib infection by the age of 5; 60 percent of those children had meningitis, and 3 to 6 percent died. (Aside from meningitis, the bacterium can also cause other serious illnesses, including pericarditis [inflammation of the sac surrounding the heart] and pneumonia.)

The Hib Vaccines

Both physicians and parents have therefore welcomed the availability of effective vaccines to combat Hib. There have been a number of such vaccines; the earliest ones were different from those in current use.

The first Hib vaccine, known as Haemophilus b polysaccharide vaccine, was licensed by FDA in April 1985 (polysaccharide refers to a viral component used in preparation of the vaccine). Within a year, a total of three manufacturers had been licensed to produce such vaccines, variously known as b-Capsa-1 (Praxis Biologics), Hib-imune (Lederle), or Hibvax (Connaught Laboratories).

Trials had demonstrated the polysaccharide vaccines' effectiveness in children at least 2 years old, but not under 18 months; their efficacy in infants between those ages was in question. The official recommendation of CDC's Immunization Practices Advisory Committee (ACIP) was for routine vaccination at 24 months, with possible use in children as young as 18 months if they were considered to be at high risk of infection.

In December 1987, a new vaccine — known as a conjugate vaccine, a technical reference to a new method of formulation — was licensed by FDA for children 18 months of age or older, in whom it was clearly more effective than the polysaccharide vaccines; one year later, a second conjugate vaccine was licensed for the same age group.

A third such vaccine was licensed in December 1989, with the statement that it could be administered to children as young as 15 months. The following spring, ACIP revised its recommendations, advising routine immunization of all children at the age of 15 months, using any of the three conjugate vaccines.

Late in 1990, after further clinical testing, FDA announced approval of two of the three vaccines for babies as young as 2 months of age. This was especially good news, since about two-thirds of all cases of Hib disease have struck children under the age of 15 months.

Vaccination against Hib infection is now considered part of the routine childhood immunization schedule, and is accomplished with the following conjugate vaccines:

HbOC (HibTITER), made by Praxis Biologics, distributed by Lederle Laboratories

PRP-OMP (PedvaxHIB), made by Merck Sharp & Dohme

PRP-D (ProHIBit), made by Connaught Laboratories, only for children at least 15 months old.

Hib vaccine can be given during doctor visits at the same time as other routine protectives — DTP (diphtheria-tetanus-pertussis), OPV (oral polio vaccine), and MMR (measles-mumps-rubella) — and all children under 5 years should receive one of the vaccines. For schedules, see "Hib Vaccine Recommendations."

No serious adverse reactions to any of the three vaccines have been reported. Minor reactions, which occur in only 1 to 4 percent of children, may include a slight elevation of temperature and a bit of redness and/or swelling around the injection site.

Most older children and adults do not need Hib immunization, and the ACIP has not issued specific recommendations except for small children. The committee cites studies, however, suggesting good immunogenicity (development of antibodies) in high-risk people given Hib vaccines. They include those with sickle cell disease, leukemia, and human immunodeficiency virus (HIV) infection, as well as persons who have had their spleens surgically removed.

Tags: Disorders and Diseases

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