By Matt McGrath
BBC News science and environment reporter
The search for an HIV vaccine has proved frustrating
A review of a trial of an HIV vaccine in Thailand has concluded that it does show real signs of a protective effect.
Scientists announced last month that a combination of vaccines gave a 31% level of protection in trials among 16,000 heterosexuals aged 18-30.
Doubts had been raised about whether the finding was significant.
But new data published at a conference in Paris indicates that, while small scale, the findings are robust and statistically significant.
However, the scientists say they do not understand what is causing the effect, and it seems to offer less protection to those most at risk.
Despite these drawbacks, Col Nelson Michael from the US military HIV research programme said it was still a small step in the right direction.
He said: "It's important that people understand that this is a scientific advance, a scientific breakthrough.
"It is not a public health breakthrough; there is not a vaccine that is around the corner.
"We now, after 26 years of trying, believe that we can go down that road with confidence that we will be able to develop a vaccine that is globally effective."
Despite their initial joy when the first results from the Thai trial were announced, many scientists were concerned that the protective effect was very small.
Among the 8,000 or so who received the vaccine, 51 became infected with the virus.
Among those who had a placebo, 74 got the infection.
However, the trial's sponsors, the US military and the Thai government, said the 31.2% protective effect was statistically significant and real.
As more data slowly emerged some scientists began to have doubts about how robust this effect really was.
When those who had not taken all their vaccines were excluded from the trial the numbers lost their statistical power.
Many experts were concerned that the data had not been subject to a rigorous examination by other scientists.
Now that review has taken place. And it indicates that the original findings still stand - but there are more questions than answers.
The scientists based their conclusions on an analysis of the entire population in the study, excluding just seven people who were found to be infected on the first day of the trial.
The researchers say this is the most robust way of looking at the study.
They also published details of two other analyses of the trial, which excluded people who had not taken all their medication.
Both of these showed effects that were not statistically significant.
Among the scientists at this meeting there was a real sense of excitement about the findings.
Dr Nicola Frahm, associate laboratory director for the HIV Vaccines trials network in Seattle, Washington, said: "There is hope. There is a vaccine that prevented acquisition, the signal is very weak but this is the first time to show any effect on acquisition at all, so for the HIV community it's an amazing day.
"The real question is to how biologically significant is this?
"The significance that has been established in this trial is that there is a 5% chance that this is a fluke.
"So we are 95% certain that what we are seeing is real and not down to pure chance. And that's great."
No use in Africa
The researchers point out that this vaccine would not work in Africa where the rates of HIV infection are the highest.
They say that a commercial product is still many years away and caution that prevention and education are the best protection against the infection in the medium term.
Col Jerome Kim is HIV vaccines product manager for the US Army.
He said: "We had something that protected a third of the people exposed to the virus in Thailand.
"It is not a globally effective vaccine; it can not be used outside of Thailand.
"Until we answer those other scientific questions the idea of using this vaccine in other parts of the world or testing it, is science rather than public health."
Deborah Jack, chief executive of NAT (National Aids Trust), said: "The Thai trial is a milestone in the search for a vaccine against HIV.
"These results are an incredible opportunity for scientists to discover new clues about HIV and learn how a HIV vaccine could work in practice."