Different types of the bacteria carry different polysaccharides, so that infection with say meningococcus type B does not lead to immunity against the C strain of the organism, or vice versa. The first vaccines produced against agents like the pneumococcus and the meningococcus therefore contained mixtures of polysaccharides extracted from different organisms. These vaccines are quite effective in adults, but their effects are fairly short-lived, and they are completely ineffective in infants less than two years old. The immune system of infants is, for reasons that are not completely understood, incapable of producing good immune responses to polysaccharides. In contrast, infants can respond well to proteins of microorganisms, such as tetanus toxoid, one of the components of the DPT triple vaccine.
Immunological memory induced by a good vaccine can persist for many years.
In general, microbial proteins produce excellent immunological memory, whereas polysaccharides do not. The strategy adopted in attempts to produce more effective vaccines against bacterial meningitis were based entirely on the results of experiments performed in mice. The immune responses of mice to bacterial polysaccharides are remarkably similar to those of humans, and scientists could study them in detail under carefully controlled laboratory conditions. This led researchers to investigate the possibility of enhancing the effectiveness of meningitis vaccines by attaching the bacterial polysaccharides to a microbial protein, producing a so-called conjugate vaccine. The first conjugate vaccine against meningitis, the Hib vaccine, was included in the childhood vaccination programme in the UK in the early part of this decade, and has led to almost complete eradication of meningitis caused by H. influenzae .