The reason we don't have an HIV vaccine is because HIV embedds itself in the DNA of some cells and some of those cells don't start producing virions right away, the immune system can't detect them, you clear the visible infection, but just weeks after, those hidden cells start producing the virus and the infection starts over.
So the issue with HIV is the hidden reservoir of infected cells, afik.
The reason we don't have an HIV vaccine is because HIV embedds itself in the DNA of some cells and some of those cells don't start producing virions right away, the immune system can't detect them, you clear the visible infection, but just weeks after, those hidden cells start producing the virus and the infection starts over. So the issue with HIV is the hidden reservoir of infected cells, afik.