Bacterial Meningitis : Unresolved Questions

by Food and Drug Administration (FDA)

(Page 3 of 3)

The use of three Hib vaccines has caused some confusion (for most other routine childhood immunizations only a single vaccine is used).

Only two of the three Hib vaccines are recommended for small babies — and they are to be given on different schedules. "There are differences in antibody response between the two vaccines approved for 2-month-olds — how soon antibodies appear, how high their levels go, how long they last," explains Bascom F. Anthony, M.D., director of the division of bacterial products of FDA's Center for Biologics Evaluation and Research. "But they are both highly effective, and both have been judged worthy of licensure. If something good comes along, and it's both safe and effective, it's likely to be approved."

The vaccines are given on different schedules, Anthony says, because approval is based on the way studies supporting a maker's claims were conducted: "The efficacy data supported the two schedules. But this," he notes, "had no influence on the timing of the booster dose; many children in the studies simply weren't old enough for a booster dose when the study ended. The booster recommendation is based on antibody levels. The evidence shows that when the PRP-OMP vaccine is given at 2 months and 4 months, antibodies may be dangerously low by 12 months, and so a booster is recommended at that age."

This issue and others were addressed at a recent National Institutes of Health workshop on Hib vaccination. A major question is whether or not the two vaccines approved for infants under 15 months are safely interchangeable — that is, if one brand is given at 2 months, can the same baby be given the other at 4 months? (Since the answer isn't known, it's now recommended that the same brand be used.) The workshop participants agreed that further research is needed, and studies to clarify the interchangeability and scheduling issues are being designed.

Meanwhile, all children should be receiving Hib vaccine. It is hoped that Hib meningitis will soon take its place in the medical history books, along with polio and diphtheria, as a disease unfondly remembered and, thankfully, rarely seen.

Bacterial Bad Guys

Although meningitis can be caused by viruses, in the critical, potentially fatal form of the disease, the villains are bacteria. The first three account for about 80 percent of all cases.

Haemophilus influenzae type b ("Hib") is the most common cause of bacterial meningitis in the United States. Other strains of the bacterium — types a, c, d, e, and f, as well as "untyped" varieties — cause less serious illness, including middle-ear infections in children. There are now vaccines to protect infants and preschoolers, Hib's most frequent victims (most older people have developed immunity). Hib can also strike elsewhere in the body, causing pneumonia and other conditions.

Neisseria meningitidis, also known as meningococcus, may strike at any age and has been epidemic at times, in some parts of the world and under closed-population circumstances, such as barracks. There is a vaccine, but it hasn't been shown to be effective in very young children or proved safe during pregnancy. It's believed to be effective for three years, but this hasn't been definitely established. The vaccine is not considered necessary for routine use in the United States, except for the military. Vaccination is, however, recommended for those who travel to sub-Saharan Africa during the December-through-June dry season, when meningococcal meningitis has been known to be epidemic there; the hazardous area is a band covering most of Ethiopia and stretching westward through Sudan, Chad, Niger, and Nigeria to Guinea on the continent's west coast.

Streptococcus pneumoniae, also known as pneumococcus, has an attack rate lower than that of H. influenzae and N. meningitidis, but the disease is just as serious. This bacterium is the main cause of meningitis in adults, although it can and does attack children as well. The meningitis is sometimes secondary to another condition caused by the same organism, such as pneumonia or an ear or sinus infection.

Other Streptococcus species, Escherichia coli, and Staphylococcus aureus are among a variety of common, widespread bacteria that are unlikely to cause critical illness in most children and adults, since most have developed immunity to them. They can, however, cause life-threatening meningitis in infants during the first four to six weeks of life, because newborns may not have protective antibodies. Low-birth-weight babies are most susceptible, and the mortality rate is high. Those with immune systems weakened by such conditions as AIDS are also vulnerable to infection by some of these agents.

Hib Vaccine Recommendations

Three vaccines are currently licensed by FDA to protect children against infections caused by the bacterium Haemophilus influenzae type b (Hib). Prominent among those infections is bacterial meningitis, of which H. influenzae is the major cause.

One vaccine, PRP-D, has been approved only for children at least 15 months old. For younger infants, either of the others may be used; since it's not known whether or not they are safely and effectively interchangeable, physicians recommend that when a child is receiving two or more shots before the age of 15 months, the same product be used.

About the Author

www.fda.gov
FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.