HIV has been a very difficult target for a vaccine for a variety of reasons. It's designed to evade the immune response by evolution, said Dr.Louis Picker, associate director of Oregon Health and Science University. While attempts to make an HIV vaccine from a dead virus have failed, Picker said, using a weekened virus holds clues and possibilities when used in primates. The live attenuated approach actually was shown to work 20 years ago. But the problem with it was the live attenuated vaccines that actually worked were actually still pathogenic. So they wrern' t safe and they could not be moved into humans. This was demonstrated in the non-human primate models-mokey modle of AIDS using the virus SIV, he said.