There is no placebo group, it would be highly unethical to do so. Half the sample population gets the treatment half of it doesn't with the option to get the treatment once trial finishes. Changes in behavior due to treatment are accounted via survey pre-treatment and at measuring intervals.
You're mistaken: "Phase III would test the effectiveness of the vaccine in a larger group of about 6,000 volunteers who are at risk of becoming infected. Half would be given the vaccine and half given a placebo. Participants would then be tracked for three years to see how many in each group become infected." - http://www.ctv.ca/CTVNews/Health/20111220/canadian-hiv-vacci...
But more interestingly, why would it be "highly unethical" to do so? They're practicing informed consent, so everyone who gets a shot knows that there's only a 50% chance that they got the real vaccine. And they know that the vaccine isn't guaranteed to work (quite the opposite, in fact, based on the track record of similar trials here in the States).
"There is no placebo group" vs "Half the sample population gets the treatment half of it doesn't"
The "doesn't" is the placebo group.
And since every single other HIV vaccine has failed, it's likely that this one will also - so it's not like they are withholding an effective treatment and acting "highly unethical".
