Polyosides (encapsulated bacteria).

The polysaccharide capsule which surrounds bacterial species such as Haemophilus influenzae, Streptococcus pneumoniae, Neisseria meningitidis and Salmonella typhi is a potent virulence factor by protecting the bacteria from phagocytosis. The host responds with antibody production and specific antibodies plus complement binding to the capsule facilitate opsonization of the micro-organism, which is phagocytized and eliminated. Purified capsular polysaccharides elicit T-independent antibody responses without a memory function, but are often poorly immunogenic in infants where much of the invasive H. influenzae type b (Hib) and pneumococcal infections is seen. Therefore purified polysaccharides have found limited use as vaccines. However, covalent linkage of the capsular polysaccharide, or fractions thereof, to immunogenic carrier proteins creates glycoconjugates which are T-dependent antigens and which elicit antibodies also in infants and which prime for boosting either with the glycoconjugate or the capsular polysaccharide. In the last decade Hib glycoconjugate vaccines have been successfully introduced and in countries with very high immunization coverage the disease has been virtually eliminated and a decline of over 95% has been seen in countries with slightly lower vaccine rates. World-wide use of Hib glycoconjugate vaccines offers the possibility of elimination of invasive Hib disease. Pneumococcal (11 serotypes with coverage of approximately 85% of invasive disease), meningococcal (A, C, W 135, Y but not B) and S. typhi glycoconjugates are in advanced development and offer the prospect of being as successful as the Hib glycoconjugates.