Picker said, "The unique aspect of the live attenuated vaccines that seem to work was that they were persistent. They weren't cleared by the host immune system, but they were able to stick around. Of course in the case of HIV/SIV that's a bad thing, because eventually those live attenuated vaccines would gain strength and cause disease. But probably the fundamental reason, at least what i hypothesized, that they would be able to elicit protection was because of that persistence" So, they looked for another petsistent virus that was not pathogenic, which might generate T cell immune response.