In a workshop at the International AIDS Conference, they said the AIDS pandemic would only be defeated by a preventative vaccine, rather than treating people who are already infected.

But they admitted there have been many setbacks in crafting such a shield, and some advocated a return to fundamentals, and said it is time to draw lessons from failure.

"Vaccine science is still more of an art than a science," said Tachi Yamada, executive director of the Bill and Melinda Gates Foundation's Global Health Program, a major donor to the vaccine effort.

Yamada pointed to fundamental gaps in knowledge about how the stealthy human immunodeficiency virus (HIV) subverts the immune system.

There also is a lack of key lab tools, including the right animal models for testing a candidate vaccine. Researchers had to rethink their approach on selecting which candidate should then be submitted to the long, exhausting three-phase trials on humans.

Meanwhile, Yamada said funding, collaboration and cooperation urgently had to be stepped up to avoid wasted or duplicated effort.

He said the arena must be opened up to smart, revolutionary ideas, and a new generation of vaccine researchers must be groomed.

"We need big investments for the future ... not only in the basic science of HIV prevention, but also in clinical trials for an HIV vaccine," Yamada said.

"We have to be unafraid to fail. ... The road to success begins with setbacks."

Alan Bernstein, director of the Global HIV Vaccine Initiative, said the hunt was "at a critical crossroads" and any success could only be viewed as "long term."

"We have to be unafraid of failure. Science is not a straight line," said Bernstein.

AIDS first emerged in 1981. Swift progress in identifying the virus that caused it unleashed early optimism that, like polio, measles and other viral threats of the past, a vaccine would quickly emerge.

To date, more than 25 million lives have been claimed by AIDS and 33 million people are estimated to have HIV.

A safe, effective primer of the body's defences -- the frontline antibody troops and the heavy artillery of the immune cells -- remains far out of reach, however. Out of the 50 candidates that have been evaluated among humans, only two vaccines have made it through all three phases of trials, and both were rejected as quite ineffective.

In the past year, one major vaccine trial was halted after early results showed that it appeared to place volunteers at greater risk of HIV infection.

Another vaccine in planning phases, involving tests among 8,500 subjects, was scrapped after a similar formula, tested in 2007, was found to be largely ineffective.

Despite the failures, about 30 vaccines remain in the pipeline, looking at different ways of stimulating the immune system by presenting harmless parts of the AIDS virus as a threat, and of delivering by various means.

Another problem-riddled mission on the HIV prevention front has been to devise a microbicide -- a gel that would kill or block the virus in the vagina. Researchers said such a product, delivered in a gel applicator, vaginal ring or capsule, could be a godsend to African women facing the threat of coercive sex by an infected partner.

There have been nine fully-completed or halted trials of microbicide candidates, one of which showed that the prototype actually boosted the risk of infection by causing vaginal lesions that helped the virus to enter the bloodstream.

Zeda Rosenberg, chief executive officer of the International Partnership for Microbicides (IPM) said the new tack is to look at a gel that incorporates virus-killing drugs that are already tried-and-tested among people with HIV.

Five gels in this category are in the early stages of trials.

Despite past failures, "there is a true cause for optimism in the field," Rosenberg said.

Using antiretroviral drugs as a "pre-exposure" prevention is another strategy being tested being in Peru and Ecuador, and set to be amplified next year.

The idea is to have people at risk from infection take one pill, or a combination of them, before sex to prevent infection.

But some researchers worry about this, fearing this start-stop use of antiretrovirals will help the virus to mutate, in the same way that incorrect use of antibiotics can help a germ to build resistance to the drugs.

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