A preliminary analysis of an Australian-developed HIV vaccine has produced disappointing results and the likely cancellation of a planned second study.

The experimental vaccine is the first to use a ‘prime-boost’ approach to preventing infection, whereby subjects are injected with not one, but two different vaccines — the first a DNA vaccine and the second a modified fowlpox virus. Researchers from the Australian/Thai Vaccine Consortium had been hopeful that the ‘double whammy’ of two different vaccinations would mean the Australian vaccine would succeed where others have failed.

The trial among 24 HIV-negative volunteers was designed to test the safety and immunogenicity (ability to generate an immune response) of the vaccine, which had shown considerable promise in animal studies. Researchers also planned a much larger efficacy trial to be run in Thailand. The consortium’s work was largely funded by a $27 million development grant from the US National Institutes of Health.

Fifteen men and nine women were enrolled in the Sydney trial, and randomly assigned to receive either the vaccine (18 participants) or a placebo (the remaining six). Participants each received three injections — two of the DNA ‘prime’ and one fowlpox ‘boost’.

The interim analysis, looking at results up to the 12-week point, found no significant problems with the vaccine’s safety, but also showed very poor immune response. Just four of the 18 participants in the vaccine arm, and one in the placebo arm, showed signs of an low-level immune response.

The vaccine’s poor immunogenicity means that the widespread trial in Thailand will not proceed, however the research team are hoping to conduct a much smaller Thai trial next year, in which another small group of volunteers would receive the vaccine a doses up to six times that used in Sydney. If the results of that trial were more encouraging than the Sydney trial, the wider Thai trial could yet be resurrected.

While the researchers say they are disappointed with the results, they remain hopeful that further studies will either produce better results, or help them understand why the vaccine seemed to work in monkeys but not in humans.