Imperfect vaccines and the evolution of pathogen virulence

Vaccines rarely provide full protection from disease. Nevertheless, partial ly effective (imperfect) vaccines may be used to protect both individuals a nd whole populations(1-3). We studied the potential impact of different typ es of imperfect vaccines on the evolution of pathogen virulence (induced ho st mortality) and the consequences for public health. Here we show that vac cines designed to reduce pathogen growth rate and/or toxicity diminish sele ction against virulent pathogens. The subsequent evolution leads to higher levels of intrinsic virulence and hence to more severe disease in unvaccina ted individuals. This evolution can erode any population-wide benefits such that overall mortality rates are unaffected, or even increase, with the le vel of vaccination coverage. In contrast, infection-blocking vaccines induc e no such effects, and can even select for lower virulence. These findings have policy implications for the development and use of vaccines that are n ot expected to provide full immunity, such as candidate vaccines for malari a(4).