BY ANDREW M.POTTS

An experimental HIV vaccine trialled by the Thai Government and the US military may be less effective than first reported, following the results and some additional analyses being published in The New England Journal of Medicine.

The full results still report an overall protection rate of 31 percent, but poorer results were found among the few men who have sex with men (MSM) involved in the trial and those from other high-risk groups, such as drug users and men who use prostitutes.

Among those who fell outside the study’s core sample of non-drug-using heterosexuals, the protective effect of the vaccine appeared to halve.

The study used two pre-existing vaccines found to be ineffective by themselves. When used together, however, infection rates among participants dropped.

Over a three-year period, of 8197 people who received the vaccine, 51 contracted HIV compared to 74 out of a control group of 8198.
The vaccine also appears to be most effective in the first 12 months, with the protective effect waning over time.
During the first year the effect seemed to be as high as 60 percent protection, however, by 30 months this had fallen to 36 percent, and by the end of the three years had averaged out to 31 percent.

Two accompanying analyses included in the report and presented at a conference in Paris last month put the effectiveness of the vaccine at closer to 26 percent using different criteria.

The executive director of the Australian Federation of AIDS Organisations, Don Baxter, told Southern Star the trial had value in pointing to how future trials could be designed better but that very little could be taken from the results at this stage.

“There are some serious confusions in the way these trial results have been released, which is really unfortunate,” Baxter said.
“Future trials need to be designed with sufficient statistical power [so] that results for subpopulations can be assessed much more accurately.

“There were very few gay men and MSM in the trial, and some who were defined as high risk from a range of other characteristics — having many sexual partners or having paid for sex for example — so we’re talking about very small numbers to be making any firm view on a candidate vaccine for which the efficacy is already very low — whether that’s 30 percent or 20 percent.”

ACON CEO Nicholas Parkhill told Southern Star, “While the initial data from the Thai vaccine study was reasonably promising, what the revised results show is that a lot more work needs to de done before an effective vaccine for HIV is developed.”

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