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when antibiotics were introduced in the late 1940s. However, therapeutic vaccination today remains the standard of care for those believed to be infected with rabies and for babies born to mothers infected with hepatitis B virus. In diseases against which therapeutic vaccination does work, the vaccine retards early infection before the development of a natural immune response.

Within the context of HIV, an injected therapy consisting of synthetic HIV antigen (e.g., gp160) is administered to people who already have HIV. It is supposed to heighten and broaden the immune response to HIV, helping to halt disease progression. Since general immune stimulation via IL-2 or other agents has not offered much hope of directly restricting HIV, researchers all along have been considering ways to construct a strong specific anti-HIV immune defense. Therapeutic vaccines are the major strategy that has been proposed to preferentially increase surviving CD4 cells that could orchestrate a new defense against HIV.

After introduction of new foreign proteins (antigen) to the body, naive cD4 cells sensitive to portions of that protein eventually are activated and multiply to construct new immune defenses. The therapeutic vaccine concept is to improve on the immune defense against a particular already-existing infection by inoculating pieces of the infectious agent's protein presented in a way to trigger new naive cell activation. An immune-enhancing adjutant is frequently used to help this process along. In 1996 investigators presented evidence that therapeutic vaccines can reduce the extent and duration of genital herpes outbreaks in people with frequent eruptions and in infected guinea pigs.

Most prominent researchers have always been skeptical of the validity of the therapeutic vaccine approach for HIV. One of the common arguments against this approach is that vaccines are not promising because the body already sees lots of HIV antigen—adding a little extra is not likely to make any difference. There is also a serious objection concerning viral diversity: Even if an induced immune response is effective, won't HIV merely mutate to rearrange the bit of viral protein that triggers the attack? Indeed, the field of therapeutic vaccines for HIV is littered with failures, despite clear demonstrations that the vaccines provoke new immune responses against HIV. To date, the most extensively tested product has been a gp160 (HIV envelope protein) inoculant made by bio-engineered insect cells, developed by Micro-GeneSys. Studies showed that the MicroGeneSys vaccine, though clearly "immunogenic," completely flopped in a placebo-controlled trial. After following 608 volunteers (with starting CD4 counts of at least 400) for three to five years, researchers concluded that the bimonthly injections of the vaccine were safe, but no difference existed between the placebo and vaccine groups in terms of occurrence of opportunistic infections, drop in CD4 count, rise in plasma HIV levels, or other measures of disease progression. A similar 278-person, 3-year Canadian trial had equally negative results. Findings of both trials were presented at the Eleventh International Conference on AIDS, July 7-12, 1996.

Three other, more naturally structured vaccine products (Immuno AG's gp160 produced by mammalian cell cultures, a hybrid canary pox virus with a gp160 envelope, and British Biotech's virus-like particles consisting of yeast protein particles coated with HIV p24 core protein) have also failed. Nonetheless those interested in this approach remain hopeful.

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