Recent developments in bacterial conjugate vaccines.

Study of the epidemiology of childhood infection reveals that the brunt of disease for a number of invasive bacterial infections is borne by children under the age of 4 years. Haemophilus influenzae type b (Hib), Neisseria meningitidis and Streptococcus pneumoniae, the three most important causes of childhood meningitis, illustrate this phenomenon, which is caused by the inability of infants and young children to mount antibodies to the carbohydrates that form a capsule surrounding these organisms. Carbohydrates are traditionally viewed as T-independent antigens with a number of unique and important immunological properties that are not encountered when inducing an immune response to proteins. These properties include no overt requirement for the presence of T cells to induce an immune response, dominance of IgM, failure to induce memory following immunisation, an absence of affinity maturation following immunisation, and poor immunogenicity in infants, the elderly and the immunocompromised. These properties of carbohydrates have precluded the use of pure carbohydrate vaccines in those patients most at risk. Conjugate vaccine technology, where a carbohydrate antigen is coupled chemically to a protein carrier, has overcome the limitations of carbohydrates as vaccine antigens by rendering the carbohydrate moiety of such vaccines immunogenic, even in the very young. The dramatic success of the Hib conjugate vaccines, the first conjugates licensed clinically for human use, in reducing the incidence of invasive Hib disease has demonstrated the potential value of such conjugate vaccines. Similar technology is, therefore, being applied to a number of other vaccines in development, including N. meningitidis (groups A and C) and S. pneumoniae vaccines. The large number of pneumococcal carbohydrate serotypes that require inclusion in a vaccine makes this conjugate formulation far more complicated than that for Hib, and it is likely that the dramatic success of the Hib conjugate vaccines will be more difficult to repeat for the pneumococcus.