For years the idea of a therapeutic vaccine for people with HIV was little more than a pipe dream, but as of 2002 there are two in the pipeline ready for human tests. Top-billed is a Merck star that targets gag, a protein once thought to be HIV's Achilles' heel and still viewed as a key to cellular immune response (see "Prime Time," POZ, May 2002). A featured player in the vax drama is the tat-based product -- tat is a protein that serves as the on-and-off switch for HIV replication -- being developed by Robert Gallo, MD, in conjunction with longtime collaborator Daniel Zagury, MD, of Paris' Institut Pasteur.

In contrast to the Merck gag-based vax, Gallo's tat evolved into a vaccine for the already-infected apparently by default -- it simply didn't fare well in preventive studies. The tat protein has long been considered a logical target because it is expressed early in HIV's life cycle, so disabling it might be an efficient way to block the virus' development. But even when the vaccine boosted immune responses to HIV, it was a bust as an infection fighter.

The human tests will run for five months, with 32 HIVers with undetectable viral loads getting tat in a series of jabs. They will remain on HAART during the trial, which is primarily designed to establish the vaccine's safety and ability to stimulate an appropriate immune response.