The Chief Executive of VaxGen, Lance Gordon, is reported to have said that a proven efficacy of just 20% would be enough for the company to file for US FDA approval of their experimental HIV vaccine, AIDSVAX, once results from the first efficacy trial are analysed, "within the next couple of months".
These remarks - to a conference of healthcare investors - were linked in at least one report (Reuters) to the better-than-expected retention of volunteers in the study (95% rather than 85%) and an HIV incidence rate in the community at large of 3% rather than 1.5%. (It isn't yet clear whether this applied directly to trial volunteers, or over what period, although it has previously been reported that incidence within the trial is high enough to guarantee a result, one way or the other.)
When the trial was first designed, the aim was to be able to distinguish betweeen a product that was ineffective and one that had 30% efficacy. A more "powerful" trial means that a narrower difference can be detected, such as between an ineffective vaccine and one with 20% efficacy.
However, if the trial is more powerful than anticipated, this also means that "confidence intervals" - the gap between the best and worst interpretations of what the trial results mean - would become narrower, not wider. Thus, if the apparent efficacy of the vaccine were 50% or 60% , then the minimum efficacy (lower limit of the confidence interval) should be higher than 30%, not reduced as low as 20%. The apparent efficacy would be the "headline" result of the trial, showing the probability of protection when the vaccine is used in a similar population (along with other HIV prevention measures).
If the minimum efficacy were as low as 20% and coupled with a low apparent efficacy, say 30%, then it is clear that there would be greater reluctance to use or recommend the vaccine, though the case for further research would be overwhelming.
As previously reported here the results of the first AIDSVAX trial are due to be announced by the end of March.