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[PARIS] The first successful trial of an HIV vaccine — announced to wide publicity last month — has shed crucial light on what happens in the body when the virus invades it, a conference has heard.

The vaccine, tested on 16,000 volunteers in Thailand, cut the rate of HIV infection by just under a third (see Breakthrough HIV vaccine first to cut infection rates).

Experts at the Aids Vaccine 2009 conference in Paris yesterday (20 October) pointed out that the vaccine only works in the Thai region and that, even there, a useable AIDS vaccine is still years away. But they are excited that the trial has revealed for the first time how the human body fights HIV infection.

With diseases such as hepatitis, scientists know which substances are produced when the body fights the virus — if they want to test a vaccine, they just need to observe whether it triggers production of the substances in a patient.

But with HIV such responses — known as host response correlates — have remained elusive, rendering it impossible to test a vaccine's effect, until now.

"The establishment of such correlates is the central question in HIV vaccine development and will have a profound effect on the designs of vaccines and clinical trials to assess their efficacy," says an editorial in the New England Journal of Medicine (NEJM).

The results of the trial were published in the NEJM this week (20 October). But preliminary data were made public one month ago, as part of a commitment by the Thai government that its people would be the first to learn the outcome of the trial.

"This is the first direct demonstration that it is possible to avoid the infection through a vaccine," Mauro Schechter, professor at the Federal University of Rio de Janeiro, Brazil, and a former AIDS vaccine sceptic, told SciDev.Net.

But the trial has raised many unanswered questions, the authors told the conference. For example, the vaccine's power seemed to decrease over the first year after vaccination and it appears to have been more protective for people at lower risk of infection.

Researchers were also unable to determine which of the two components comprising the vaccine — known as ALVAC-HIV and AIDSVAX B/E — was effective, or whether it was the combination that was important.

The study has heightened the debate on when to start testing on humans.

"The thing is that there are issues we can only know about if we test in human beings," Schechter said. "This is especially important when we are talking about HIV/AIDS, since humans are the only ones known to be infected by HIV — thus there are no animal models."