Jeremy Laurance reports on the jab made in Thailand that cuts infection rates by 30% – and offers hope to millions

The scientific naysayers who claimed a vaccine against HIV would never be possible have received their comeuppance. After years of setbacks and growing doubts, a jab to prevent the worst disease of modern times, which currently affects 33 million people worldwide, may be in prospect after all.

The world's largest HIV prevention trial, involving 16,000 people in Thailand, reported yesterday that giving a combination of two vaccines lowered the risk of contracting the virus by 31.2 per cent. That is not enough for a viable vaccine that could be used globally against HIV but it is the first indication that an effective jab – one that provides at least a 50 per cent reduction in risk – might be possible.

The World Health Organisation and the Joint United Nations Programme on HIV/Aids hailed the result as a "significant scientific breakthrough". Seth Berkley, president of the International Aids Vaccine Initiative, said it was "very exciting news".

The advance comes after more than 20 years of research, the expenditure of billions of pounds and four previous vaccine trials with single agents which failed to demonstrate any protective effect. It is a victory for the US and Thai researchers who pushed through the $120m trial in the face of heavyweight scientific opposition. One of the chief critics was Robert Gallo, a discoverer of HIV, who had scoffed: "We'd learn more if we had extract of maple leaf in the vaccine."

Another blow came when a group of two dozen scientists wrote to the journal Science in 2004 that the inclusion of one of the candidate vaccines, made by Vaxgen – which had failed a previous trial – was "completely incapable of preventing or ameliorating" HIV infection. The group questioned "the wisdom of the US government's sponsoring" the Thailand trial.

A poll of 35 international Aids researchers published by The Independent in April 2008 revealed a mood of "deep pessimism" within the scientific community, with a substantial minority admitting an HIV vaccine might never be developed. Aids organisations had called for the funds spent on the search for a vaccine to be diverted to other prevention efforts. Now the results of the Thai trial are in, its critics may have to eat their words.

The first vaccine used in the trial, called ALVAC, is based on a canarypox virus that has been disabled which is used as a "Trojan horse" to smuggle three genetic fragments of HIV into the body, priming the immune system to recognise and kill HIV-infected cells. The second vaccine, AidsVAX, contains a protein designed to encourage the body to produce neutralising antibodies to destroy HIV before it infects healthy cells. The researchers warned that the two vaccine components might not work in other parts of the world, such as sub-Saharan Africa, where different strains of HIV are circulating and the population has a different genetic make-up.

Thailand was chosen 18 years ago by the WHO for HIV vaccine trials that were then thought to be imminent. Scientists predicted that a vaccine to prevent the infection would be ready long before a treatment for the symptoms could be developed but the opposite turned out to be true.

Millions of people are keeping the virus under control with drugs. But these are not a cure. In contrast to virtually every other microbe known, there is no documented case of anyone who has ultimately cleared the virus from their body completely. That is why developing a vaccine to prevent the infection has been a priority.

The trial, funded by the US Army and the US National Institute for Allergy and Infectious Diseases, was run in Thailand because it is a country with a high rate of HIV, with a good health infrastructure and the Thai government was willing to host it.

Sheena McCormack, senior clinical scientist at the UK Medical Research Council, and the African-European HIV Vaccine Development Network, said the results of the Thai trial were statistically significant. "This is encouraging. It is proof of concept and strongly suggests we may be able to achieve a vaccine. It will help us select and design vaccine candidates for the future," she said.

The next stage would be to examine the immune response generated by the vaccine in the Thai trial. Each volunteer received four doses of one vaccine and two of the other – six jabs in all. Some got strong immune responses which failed to protect them from HIV infection while others with weaker immune responses were protected.

"Obviously, if the immune response is very strong but it is not protective that is no good. The next task is to analyse the immune responses. But with 16,000 participants in the trial that is a challenge," she added.

The Aids years

1981 Aids is identified as a new disease among gays in California and New York

1984 HIV identified as cause of Aids

1984 Aids is discovered to be widespread in parts of Africa

1987 AZT is licensed as the first drug to treat Aids

1990 Around 8 million people are living with HIV worldwide

1996 Combination treatment – a cocktail of three drugs – is shown to be highly effective in preventing HIV progressing to Aids

2003 The first Aids vaccine to undergo a major trial is found to be ineffective

2007 Another major HIV vaccine trial is halted after results show no benefit

2009 World's largest vaccine trial among 16,000 volunteers in Thailand shows the risk of infection reduced by 31.2 per cent