The first Aids vaccine to show an encouraging result in clinical trials is only modestly effective and did not protect those at highest risk of HIV, it emerged today, as US and Thai researchers revealed their full results in Paris.

The announcement last month that the controversial $105m (£64m) trial carried out in Thailand had unexpectedly been successful took experts by surprise and sparked excitement around the world amid speculation that the steady spread of HIV/Aids could before long be checked.

However, it is now clear that this is a long way wide of the mark.

The full results, presented at the Aids vaccine conference in Paris and published immediately online by the New England Journal of Medicine reveal that:

• The vaccine did not protect those at high risk of HIV infection, such as sex workers and intravenous drug users.

• The protective effect was greatest in the first 12 months and then seemed to diminish.

• When those who did not get all six vaccine shots were taken out of the analysis, the positive result was statistically insignificant.

Some scientists and campaigners in Paris nevertheless hailed the results, if not as a triumph, then as a beacon of hope. Dr Nelson Michael of the US army – the US military had run the research programme with the Thai government – said scientists would now be working intensely to pick up clues to future Aids vaccine development from the results of the trial. "It is a signpost for vaccine development," he said. "This was a yes-we-can moment: the opportunity to become enthusiastic. The door has cracked open. We are all going to try to collectively crash through it."

But Seth Berkley, president of the International Aids Vaccine Initiative, which evaluates and channels funds into trials, said he thought the regime tested in Thailand would be taken no further. The vaccines involved – AidsVax, which previously failed in a trial in Thailand on its own, and Alvac – were 15 years old.

"That's why there was scientific controversy about starting this trial," he said. "If you were going to start again, you might use one of the more robust vaccines we have [in the lab] today.

"We don't have a vaccine on the horizon. It isn't the Thai one or one of the others. That's why we have to have the patience for this marathon rather than a sprint."

The Thai trial, given the name RV144, was controversial from the start because it involved two vaccines given together, one of which had previously failed to protect people from HIV while the other had not been tested alone. Many people felt the $105m it cost could be better spent.

More than 16,400 Thai men and women aged 18 to 30 who did not have HIV were recruited and randomly assigned to receive the vaccine or a placebo. They were given six shots over six months: two of Alvac followed by two more sessions where they were injected with both vaccines. They were tested for HIV every six months, and counselled on how to avoid it for the next three years.

By the end of the trial only 125 people had become infected with HIV. In the announcement of the headline results last month, it was said that 51 of those given the vaccine had become HIV positive compared with 74 in the placebo group, which gives a statistically significant efficacy of 31%.

But many volunteers did not get all six vaccinations, taking the numbers down from around 8,000 in each group to around 6,000. Among those people, there were 50 infections on placebo and 36 on the vaccine, which gives an efficacy of 26% but is not statistically significant, meaning it could happen by chance.