An AIDS vaccine, then, must teach the immune system to fight H.I.V. in a way it does not normally do. The task is not impossible, scientists say, but it will not be easy. One difficulty will be in deciding how to tell whether a vaccine works. They may be hard to distinguish from signs of infection. And if a vaccine is given to infected people who are also taking antiviral drugs, it will be even harder to figure out what is going on.
Researchers say they know surprisingly little about how vaccines work.
Antibodies are proteins that stick to viruses and other invaders and prevent them from entering cells. Keeping viruses out of cells is an important means of defense, because all viruses, H.I.V. included, need to penetrate their hosts' cells to replicate. There are backup forces, too. The second arm of the immune system, cell-mediated immunity, includes several types of white blood cells that destroy cells that have been infected by viruses. Cell-mediated immunity eliminates viruses that manage to get past the antibodies.