Resistance to a post-infection HIV vaccine that stimulates cytotoxic T-lymphocytes (CTLs) depends on the relationship between the vaccine strength, the fitness cost of the mutant strain, and the rate of mutant escape. If the vaccine is strong enough, both strains of the virus should be controlled by administering the vaccine sufficiently often. However, if escape mutation to the vaccine occurs, then either the wild type or the mutant can out-compete the other strain. Imperfect adherence may result in the persistance of the mutant, while fluctuations in the vaccination time - even if no vaccines are missed - may result in the mutant out-competing the wild type.