6.5 Multiple strains
A similar problem for vaccine design comes from those pathogens that exist in a very large number of different ‘strains’ which are in circulation concurrently. The individual strains are generally stable, so it is not the case (as with rapid mutation and antigenic drift) that new strains are arising all the time, but the numbers of different strains that would need to be included in a vaccine is immense. For example, the streptococci that cause disease in humans can be classified into 20 main serotypes and more than 100 subtypes, based on recognition by specific antibodies. Salmonella bacteria exist in over 2200 different serotypes! This makes it very difficult to design a vaccine that can induce protection against even a significant number of these strains. Apart from the basic problem of identifying and culturing the relevant strains, if too many are included in the vaccine, then the amount of each is too low to induce an adequate immune response. At present, satisfactory vaccines exist for only a few of the pathogenic salmonellae or streptococcal bacteria.