Vaccines are one of humankind's greatest weapons against infectious diseases. However, their usefulness is often questioned and the public tends to distrust vaccines. A mathematical model published in the early 2000s predicts the selection of more virulent strains of pathogens when populations are protected with imperfect vaccines, i.e., vaccines which reduce but do not entirely block pathogen transmission, such as toxoid vaccines. In this study, we built a disease-specific competition model to analyze the evolution of diphtheria's virulence under the pressure of a toxoid vaccine. Our results show that i) high vaccine coverage favors the emergence and increase prevalence of avirulent (or less virulent) strains of Corynebacterium diphtheriae (the etiologic agent of diphtheria) and ii) that competition between strains is crucial in the eradication of toxigenic strains when toxoid vaccines are used. We conclude that the use of toxoid vaccines could lead to disease eradication if the interaction between strains is taken into account. Our results could extend to biologically similar systems such as pertussis.