Many estimate these vaccines are still a decade or more away. One reason for the new optimism is that scientists have lowered the bar. Until now, all useful vaccines prevented infections. However, the human immune system cannot turn back an HIV infection, and no one knows how to make a vaccine that accomplishes something the human body cannot do for itself. So the new vaccines are designed to accomplish the next best thing train the immune defenses to hold an infection in check without preventing it entirely.
Scientists agree that blocking an infection requires the production of powerful antibodies.
If the bug ever gets into the body, the defenses will blaze back with antibodies that latch onto the protein, blocking the germ and destroying it. HIV, however, mutates so fast it constantly changes the proteins on its surface. So a vaccine that triggers an attack against one strain of HIV may be powerless against another. Furthermore, the virus covers its surface with sugar, which hides its proteins from antibodies. But the new vaccines are designed to start the opening counterattack by killer cells more quickly, so fewer helper cells get infected, and the virus eventually plateaus at a much lower level.