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A key question here is what is the expected rela- tionship between vaccine use and the evolution of pathogen virulence? Examples of evolution of virulence include the prevalence of non-toxigenic diphtheria in highly vaccinated populations and the classical example of myoxma virus/rabbit studies of Australia. To date, most work on the evolutionary effects of vaccination has focused on escape mutants, but recently there has also been some influential work done of virulence evolution as well. Important questions we will address are: (1) how do different vaccination strategies affect the expected virulence of a pathogen? and (2) how are these evolutionary changes related to the appearance and spread of escape mutants? Escape mutants are also a consequence of the high mutation rates found among most viruses which make of them a moving target from the immunological point of view. This phenomenon can be seen as one of the facets of virulence with important implications to vaccine development and use. For example, one of the recognized difficulties in developing an HIV vaccine is attributed to the virus diversity within and among individuals and countries. Besides, new influenza vaccines need to be developed each new season to cope with the virus ever changing composition. Another facet of virulence manifests as adverse events caused by live attenuated virus vaccines. Morbidity associated to vaccinal viruses is well described for polio and yellow fever. | |
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