One difficulty will be in deciding how to tell whether a vaccine works. They may be hard to distinguish from signs of infection. 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. The first efforts to make an AIDS vaccine followed the traditional path, and set out to stimulate the immune system to make antibodies that would prevent infection. Even though infected people do not produce effective antibodies, scientists hoped they could develop a vaccine that would do what the body alone could not.
Many scientists are more enthusiastic about advances made within the last year.
The advances involve vaccines that focus on stimulating cellular immunity, rather than just antibodies. Several research teams, one led by Dr. Emilio Emini and Dr. John Shiver at Merck, have had extremely encouraging results in monkeys that were given experimental vaccines and were then exposed to a virus that normally gives monkeys an unusually severe and rapidly progressive AIDS-like illness. The vaccinated animals became infected, but their killer T-cells managed to keep the virus in check, and they never got sick.