There are no dumb questions with this problem. What I think you are asking is if the dangerous strains could mutate enough that an immune response to the select

There are no dumb questions with this problem. What I think you are asking is if the dangerous strains could mutate enough that an immune response to the selected attenuated strain no longer work. The answer is yes this could happen, but if it does we just repeat the process of finding a new attenuated strain.