An AIDS vaccine is still years away even under the most optimistic scenario and scientists may have to go back to the drawing-board if the current batch of candidates, all focused on one approach, fail.

Seth Berkley, president and chief executive of the International AIDS Vaccine Initiative (IAVI), said Monday the number of potential vaccines in clinical trials had doubled since 2000, but the research effort remained inadequate.

“The world is inching toward a vaccine, when we should be making strides,” he told reporters at the 15th International AIDS Conference.

“Only a vaccine can end the epidemic.”

IAVI’s biennial report on the state of research - which called for a doubling of AIDS vaccine funding to $1.3 billion a year - listed more than 30 candidates now in clinical trials in 19 countries.

But only one has reached final phase III testing and many scientists are doubtful about the product, which combines Aventis’s Alvac with VaxGen’s Aidsvax, following the failure of separate Aidsvax trials last year.

Merck, meanwhile, is expected to start a phase IIb “proof of concept” trial on its vaccine before the end of the year.

In both cases, IAVI clinical research head Wayne Koff said researchers would not find out whether the vaccines worked until late 2007 or 2008.

Merck says its product is unlikely to be 100 percent effective. “We don’t expect to have a sterilizing immunity,” said company vaccine researcher Jon Condra.

T-CELLS VS ANTIBODIES

Part of the problem is that products that have advanced to the human testing stage are focused on the single concept of boosting white blood cells called cytotoxic T lymphocytes, or T-cells. The T-cells attack cells that have been infected by HIV.

But there are no vaccines in clinical development based on stimulating antibodies to fight the virus - an approach used in many traditional vaccines but which has so far proved extremely difficult for HIV.

“Even though we have widened the pipeline, almost all the candidates are working on a single approach,” said Koff.

Since cell-mediated immunity does not actually prevent infection, but only deals with it after cells have been invaded by virus particles, many researchers doubt it can ever be a complete barrier to HIV.

Nonetheless, a partially effective vaccine could still play an important role in reining in the virus, which has infected about 38 million people worldwide and which is growing by 14,000 new infections a day, according to Berkley.

He estimates a 50 percent effective vaccine, given to two-thirds of the adult population, could reduce infections up to 60 percent.

“Even as low as 30 percent could have a dramatic effect if it was used in high-risk populations,” Berkley added.