What price vaccines?
In some cases it could be, according to a study in the 13 December of Nature that applies the evolutionary theory for virulence evolution into an epidemiological framework.
The main problem is that vaccines that reduce the growth rate or toxicity of a pathogen can also diminish selection against virulent pathogens — resulting in more severe form of the disease on unvaccinated individuals. This is relevant for strategies to control malaria as some candidate vaccines give only partial protection.
Reference: Nature 414, 751 (2001)
Link to full text