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The news headlines about the search for a vaccine to protect against HIV-AIDS have been unrelentingly negative in recent years, but that is no reason to give up hope, an international conference heard yesterday.

"The road to success is littered with failure," Tadataka Yamada, executive director of the global health program at the Bill & Melinda Gates Foundation, told the International AIDS Conference in Mexico City. "We have to be unafraid to fail."

He said that while creating an AIDS vaccine is one of the most daunting scientific endeavours ever, there can be no doubt the payoff will be tremendous.

"The challenges are huge but I have no doubt that we will live in a world without HIV some day," Dr. Yamada said.

Alan Bernstein, executive director of the Global HIV Vaccine Enterprise, expressed a similar point of view.

"Our objective should be to stop this virus, and the best way to do so is with a vaccine," he told delegates.

Dr. Bernstein said one of the biggest problems with the search for an AIDS vaccine is unrealistic expectations on the part of the public.

"We need to get away from this home-run mentality to research. Science is incremental."

Dr. Bernstein stressed that while there have been many failures - more than 100 vaccine candidates to date - much has been learned from the research.

Unfortunately, he said, too much work is being done in isolation and using out-of-date approaches.

"Vaccine research is no longer a Mom and Pop shop. It requires international consortia and a global strategic plan," Dr. Bernstein said.

Seth Berkley, president and CEO of the International AIDS Vaccine Initiative, said he also remained upbeat. While there are many biological and social challenges to creating an AIDS vaccine, he said the bottom line remains that a "vaccine is possible" and it will come sooner or later.

Dr. Berkley said that rather than be discouraged by negative results, scientists and those who support their work - private corporations and governments alike - should redouble their commitments.

Aside from a vaccine, one of the most dreamed-about products in the AIDS world is a microbicide, a spray or gel that would allow women to protect themselves against the human immunodeficiency virus as if using an invisible condom.

But microbicides have been plagued by the same problem as vaccines: a whole lot of promising theory and very little success on the ground.

"Microbicide development is a series of barriers - no pun intended," said Zeda Rosenberg, CEO of the International Partnership for Microbicides. There have been a dozen high-profile trials of promising products but none has really delivered on reducing the risk of HIV-AIDS transmission in women.

Still, Dr. Rosenberg insisted there is real cause for optimism in the field, particularly with the advent of a new approach - taking potent antiretroviral drugs (those that have been so successful in prolonging the lives of people with HIV-AIDS) and using them to block transmission of the virus.

Scientists working on microbicides are also moving away from creating products that need to be applied before sexual intercourse and creating long-acting drugs that can be released in a vaginal ring.

"We need a microbicide that is not coitally dependent. We need to get protection away from the sex act, like we've done with oral contraception for women," Dr. Rosenberg said.

Manju Chatani, co-ordinator of the African Microbicides Advocacy Group, said that, like a vaccine, a microbicide is an essential tool in the fight against HIV-AIDS, especially for women.

"Women are disproportionately infected with HIV. That's because women still have to ask permission to protect themselves against HIV," she said.

Worldwide, there are an estimated 33 million people infected with HIV-AIDS, half of them women. But in sub-Saharan Africa, where two in three of infected people live, women are contracting HIV-AIDS at a much more rapid pace than men.