Since HIV attacks immune cells, however,

Since HIV attacks immune cells, however, many researchers worry about allowing even a tiny amount of the virus to replicate in the blood. They believe that HIV's ability to linger and mutate in cells for many years would mean that it would eventually emerge and cause AIDS. Says Letvin: "What we are asking the HIV vaccine to do is to stop the virus cold."

Dr. Anthony Fauci

A vaccine that might lessen the severity of HIV infection and avoid the symptoms of AIDS is now at the forefront of research. Scientists are searching for ways to develop a vaccine that would not prevent HIV infection but rather would control the virus once it entered the body.

"I think we'll have a vaccine that will have a major impact on the epidemic in being able to slow the spread," says Dr. Anthony Fauci, head of the National Institute of Allergies and Infectious Diseases. "If you have a vaccine that keeps replication of the virus to such a low level that you have difficulty spreading it to another individual, then interruption of the acceleration of the epidemic is attainable."