MEXICO CITY, 5 August 2008 (IRIN) - The road to finding an effective HIV vaccine has recently been marked by a string of disappointing setbacks, and researchers have warned that a breakthrough in this field is still a long way off. Should we give up hope?

Delegates at the International AIDS Conference in Mexico City heard on Monday that the search for a vaccine to protect against HIV infection was one of the most difficult endeavours ever, with vaccine science still "more an art than a science".

In the past year, one major vaccine trial was halted after early results showed that not only did the candidate not protect people from the virus, it may actually have put them at increased risk of becoming infected.

Last month, the US National Institute of Allergy and Infectious Diseases (NIAID) announced it would not move ahead with a vaccine trial involving 8,500 participants, which was intended to test a candidate with similarities to the one that failed last year.

These developments have left the vaccine field reeling, with people in some quarters calling for public funding of AIDS vaccine research to be ended or diverted toward existing HIV treatment and prevention interventions.

But Tachi Yamada, executive director of the Bill and Melinda Gates Foundation's Global Health Programme, a major funder of vaccine research, told delegates that the recent failures in HIV vaccine trials were no reason to give up hope.

"This is not a time to step away and invest less...we need to move forward with big investments. We have to be unafraid to fail," he said.

According to Yamada, scientists have very little predictive knowledge of whether vaccine candidates will work. He called for better animal models for the testing of candidate vaccines, as existing models could not predict effects on humans.

Another challenge for clinical trials was that participants did not always follow instructions, making it more difficult to demonstrate a vaccine's effects.

Unrealistic expectations on the part of the public also continued to be a problem. "When a trial goes wrong, the immediate assumption is that it was a bad investment," said Yamada, who called for more to be done to educate the public about HIV prevention research.

Alan Bernstein, director of the Global HIV Vaccine Enterprise, admitted that the search for an effective vaccine was "at a critical crossroads". He told delegates that the challenge now was to design trials that would "advance our understanding of the human immune response to HIV".

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