Seth Berkley, head of the International Aids Vaccine Initiative (IAVI), said the research pipeline, which previously wheezed out tiny drips, was now becoming a small but steady flow.

"There's a lot of excitement," he said. "You've got the first data about protection. You've also got extremely potent antibodies that are showing new targets and there's a lot more of that coming, that field's exploding right now."

Berkley also noted that the two vaccines in the Thai trial were designed some 15 years ago. Smarter vaccines have since emerged, using different viral parts to prime the immune system and novel methods to deliver them.

Berkley pointed at progress ? among lab monkeys, not humans ? on so-called cell-mediated vaccines, in which immune cells are primed to clear out the Aids virus after infection.

"What is happening now has sort of revitalised optimism," said Muhammad Bakari of the Muhimbili University College of Health and Allied Sciences in Tanzania.

"If you borrow the example from antiretrovirals, people thought it would take many, many years to get these drugs but the speed was much, much faster than what was thought initially. So I think we should be optimistic."

Bakari's team reported very encouraging results from an early trial, gathering 60 Tanzanian policeman, who were given either a Swedish candidate vaccine called DNA/MVA, or a placebo.

All those who were given the primer and booster showed a very strong immune response, "as high as any" in previous vaccine trials, he said.

At this early stage, the vaccine is tested for safety, not for efficacy, and delivery and dosage may have to be modified, but the results should warrant arguing for a wider trial, he said.

French scientist Francoise Barre-Sinoussi, who co-won the 2008 Nobel Prize for Medicine, cautioned that a vaccine breakthrough still depended on answering fundamental questions about HIV and the pathways of infection.

With vaccines, "you are only looking for a single piece of the jigsaw puzzle. A single piece never gives you the whole picture. It's all the pieces of the puzzle put together that give the answer."