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6.7 Conclusion

Where pathogens show great antigenic variation or multiple strains, the problems of vaccine development revolve around difficulties in identifying critical antigens that show little variation and which induce protective immunity. The challenge to achieve this has not yet been met for a number of important infectious diseases, which still lack effective vaccines (Table 4). Nevertheless, with the possible exception of prions, there are no theoretical reasons why vaccines cannot be developed to give protection against most infectious diseases. However, the limitations on vaccination strategies extend beyond the challenges posed by incomplete biological knowledge – as the final section of this chapter illustrates

Table 4 Some examples of infections that cannot yet be controlled by vaccination.†
PathogenExamplesDiseaseProblem with vaccine design
helminths Schistosoma speciesschistosomiasisantigenic disguise with host proteins
protoctists Plasmodium speciesmalariaantigenic variation and morphological complexity
Trypanosoma speciessleeping sicknessextreme antigenic variation
fungi Pneumocystis fungal pneumoniaignorance of effective immunity
Candida thrushignorance of effective immunity
bacteria Streptococci skin and throat infectionsmultiple serotypes
Treponem a pallidum syphilisignorance of effective immunity
virusesHIVAIDSantigenic variation
‘cold’ virusescommon coldmany different types of unrelated virus
prionsvCJD prionsvariant Creutzfeldt-Jakob diseaselack of immunogenicity

Many other infectious diseases can only be partially controlled by vaccines with low efficacy (e.g. cholera).