The promise of conjugate vaccines for Africa.

Capsular polysaccharide (PS) vaccines against Neisseria meningitidis and Streptococcus pneumoniae have proven safe and effective. Moreover, experience with N. meningitidis, Haemophilus influenzae type b (Hib) and S. pneumoniae conjugate vaccines has demonstrated that immunogenicity of PS vaccines can be greatly improved by chemical conjugation to a protein carrier. These vaccines have been shown to stimulate B cell- and T cell-dependent immune responses, to induce immunological memory and to confer herd immunity. Their introduction has had a dramatic impact on the incidence of the diseases caused by these bacterial pathogens, but questions remain on the optimal schedules for immunization. The current schedule recommended by the World Health Organization (6, 10 and 14 weeks) was developed in the early 1980s and does not take into consideration the crucial role of herd immunity in interrupting transmission. A review of the evidence in order to analyse how to optimize immunization schedules for conjugate vaccines is warranted.