picker said The unique aspect of the attenuated vaccines that seem to work was that they were persisnt. They weren't cleared by the host immune system, but they were able to stick around. of course in the casevof 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 l hypothesized,that they would be able to eilcit protection was because of that persistence