Will medical intervention such as vaccination prompt the evolution of more or virulent pathogens?
The evolution of drug resistant and vaccine escape mutants degrades the efficacy of chemotherapy and vaccination programmes. But treatment evasion is not the only detrimental evolution that can be prompted by medical and veterinary intervention. Virulence and transmission traits are the targets of animal and public health programmes, and they are intimately tied up with pathogen fitness.
We have argued that medical interventions that protect the host from death will relax selection against virulence. There are also situations where more virulent strains can better escape vaccine-induced immunity. Both scenarios can prompt the evolution of pathogens that, in the medium term, put unprotected hosts at greater risk and can in some cases make the population as a whole worse off. The framework underpinning this assertion is potentially relevant to a wide range of microparasites, but the specific fitness functions are grounded in our experiments on virulence of Plasmodium chabaudi in laboratory mice, and our on going work on Marek’s disease in poultry.
For an introductory overview, see Read and Mackinnon (2008)