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
| Pathogen | Examples | Disease | Problem with vaccine design |
|---|---|---|---|
| helminths | Schistosoma species | schistosomiasis | antigenic disguise with host proteins |
| protoctists | Plasmodium species | malaria | antigenic variation and morphological complexity |
| Trypanosoma species | sleeping sickness | extreme antigenic variation | |
| fungi | Pneumocystis | fungal pneumonia | ignorance of effective immunity |
| Candida | thrush | ignorance of effective immunity | |
| bacteria | Streptococci | skin and throat infections | multiple serotypes |
| Treponem a pallidum | syphilis | ignorance of effective immunity | |
| viruses | HIV | AIDS | antigenic variation |
| ‘cold’ viruses | common cold | many different types of unrelated virus | |
| prions | vCJD prions | variant Creutzfeldt-Jakob disease | lack of immunogenicity |
†Many other infectious diseases can only be partially controlled by vaccines with low efficacy (e.g. cholera).