1. Institute of Cell, Animal and Population Biology, The University of Edinburgh, Edinburgh EH9 3JT, UK
2. These authors contributed equally to this work

Correspondence to: Sylvain Gandon^1,2 Correspondence and requests for materials should be addressed to S.G. (e-mail: Email: Sylvain.Gandon@ed.ac.uk).

Vaccines rarely provide full protection from disease. Nevertheless, partially effective (imperfect) vaccines may be used to protect both individuals and whole populations^{1, 2, 3}. We studied the potential impact of different types of imperfect vaccines on the evolution of pathogen virulence (induced host mortality) and the consequences for public health. Here we show that vaccines designed to reduce pathogen growth rate and/or toxicity diminish selection against virulent pathogens. The subsequent evolution leads to higher levels of intrinsic virulence and hence to more severe disease in unvaccinated individuals. This evolution can erode any population-wide benefits such that overall mortality rates are unaffected, or even increase, with the level of vaccination coverage. In contrast, infection-blocking vaccines induce no such effects, and can even select for lower virulence. These findings have policy implications for the development and use of vaccines that are not expected to provide full immunity, such as candidate vaccines for malaria⁴.