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Haemophilus influenzae type b is a type of bacteria that can cause serious infection, illness, and death in children under the age of five years. In addition, up to 35 percent of surviving children may have residual neurological problems. One of the most common infections caused by this organism is meningitis, an inflammation of the membrane surrounding the brain, but other infections also result from H. influenzae type b. The development of safe and effective vaccines is an important step in preventing such infections. A vaccine against sugars from the bacterial coat has been developed but is not effective in children under the age of two years (the age group with the highest incidence of infection) apparently because at that age, important white blood cells (T-lymphocytes) cannot make antibodies against sugar molecules (polysaccharides). Conjugation, or linking, of the sugars to a protein known to activate T-lymphocytes was used as a way to develop a vaccine which would successfully protect children under the age of two. Previously, a protein from diphtheria toxin was used in a conjugate vaccine, but it was not consistently effective in protecting children. Proteins from the coat of a bacterium which causes a type of meningitis have now been chosen for conjugate vaccines because in previous uses they caused few side effects, they are known to activate T-lymphocytes, and they would provide the added benefit of immunization against this meningitis. The effectiveness of this new conjugate vaccine has been compared with that of the regular polysaccharide vaccine and also the old conjugate vaccine in tests that measure antibody activity against bacterial cells and T-cell activation in mice and young monkeys. The results indicate that the new conjugate vaccine produces antibodies in mice and young monkeys and causes T-lymphocyte activation, and that the vaccine is more effective than the old vaccines. The specific way in which the polysaccharide and proteins were bonded together was an important determinant of this success. There was a stronger protective response in monkeys if the vaccines were given with aluminum hydroxide, a compound which commonly increases immune responses. The effect of the old vaccines in young monkeys resembled that found in children, which suggests that the success of the new conjugate vaccine in juvenile monkeys may indicate that it would be effective in young children as well. (Consumer Summary produced by Reliance Medical Information, Inc.)