This article is a stub. Please feel free to expand it and make it more encyclopaedic.
Many bacteria are surrounded by a polysaccharide capsule, which both provides the antigens against which antibodies can act, the molecules that largely determine how virulent and pathogenic the organism and protects the main bacterial cell from the bodies defense systems.
It is relatively easy to create polysaccharide vaccines against the molecules in the capsule; but these molecules are often small, and not very immunogenic. As a consequence they:
- tend not to be effective in infants and young children (under 18-24 months);
- induce only short-term immunity - there is a slow immune response, with antibody levels rising slowly, with no immune memory. (The lack of immune memory means that on subsequent challenge there is no rapid response - it is as if the person being challenged had not encountered the antigens before.)
Frequent re-vaccination with any plain polysaccharide vaccine is generally avoided because of a poorly-understood phenomenon called immune tolerance which occurs with polysaccharide vaccines: with each successive dose of a vaccine, there is a poorer immune response to it. It isn't clear whether this indicates that the immune response to the natural infection would also be
Polysaccharide vaccines only protect for a few years, and cannot be used in children under the age of abut 18 months. From this age their efficacy gradually increases. Group C meningococcal vaccine, for example, is of comparable efficacy to the conjugate vaccine by the mid to late teens.
- Do not induce immune memory; indeed, there is some evidence that some of them induce immune tolerance, a phenomenon whereby subsequent doses of vaccine induce poorer antibody response.
- May not prevent colonisation, e.g. of the nasopharynx by meningococci. This means that, while they provide personal protection, they may not provide herd immunity - a person can still be colonised (infected but not ill), and pass a potentially pathogenic bug on to somebody else.
Because they are relatively simple to produce, they are also relatively cheap and uncomplicated.
For these reasons they are not generally suitable for universal use. Examples of polysaccharide vaccines which are, or have recently been used, include vaccines against:
- Meningococcal disease caused by Neisseria meningitidis groups A, C, W135 and Y. Until the introduction of a group C conjugate vaccine a polysaccharide group c vaccine would be offered to contacts of cases of meningococcal disease
- Pneumococcal vaccine. This is an interesting example, because there are also conjugate vaccines available or coming on to the market; but these protect against many fewer serotypes of disease than the 23-valent polysaccharide vaccine