Hopes for AIDS Vaccine Still Alive Despite Setbacks

But a remnant of virus will still get through, infecting human cells. That's where a second arm of vaccination, called the "cell-mediated" response, comes in. This involves the mobilization of immune T-cells that seek out and destroy infected cells.

Any viable vaccine must wipe out the virus quickly and completely, Koff explained, "So, we need each of these two arms primed and ready for when you get exposed to HIV."

But there's yet one more hurdle unique to HIV. Unlike standard flu or cold viruses, HIV -- a more primitive retrovirus -- actually inserts a piece of its DNA into the genome of the human host cell.

This "integration" begins within hours or days of infection, mainly in the lymphoid tissues of the gut, and lasts a lifetime. Using these cells as a base, HIV can then amplify and mount an attack at any time.

"This means that both antibodies and activated T-cells will have to be [targeted] to where the virus is amplifying early," Koff said. "If we don't get it then, and it seeds other lymphoid organs in the body, then from a vaccine point of view, the party's over."

So, those are the main components of any viable vaccine: a broad immunity against a variety of strains; a two-pronged immune attack that involves antibodies and T-cells; and a "hit-hard, hit-early" approach that stops HIV from setting up it's stronghold in the gut.

And, as Gallo, stressed, "any immune response must last, because we can't vaccinate every couple of months."

The task ahead seems daunting, but progress is being made, the experts said.

According to Gallo, his lab at the University of Maryland's Institute of Human Virology has one promising candidate. In primate trials, the vaccine has fulfilled most of the criteria -- except that immunity seems to fade with time.

"If we can solve the problem of keeping immunity going long-term, I'd say it's a candidate-rational vaccine," Gallo said.

Over at the University of Wisconsin, Madison, a group led by pathologist David Watkins has successfully tested a vaccine that has provided monkeys with protection from up to 20 different strains of the simian equivalent of HIV, using only T-cell mediated immunity.

Similar experiments have panned out well in pathologist Louis J. Picker's lab at the Oregon Health & Science University, Koff noted.