AIDS and Statistics

Results from AIDS tests in Thailand have been making the rounds in the news. I am a bit baffled by the statistics. Apparently 16,000 people from the general population were divided in two groups. Half were vaccinated while the other half received a placebo. They were then given condoms, taught how to prevent infection, and promised treatment in case the contracted the disease. The result that has been making the news is that 74 in the placebo got infected, while only 51 in the vaccinated group contracted the disease. Now, there is something that does not compute here. Couldn't we explain the difference simply by hypothesizing that 51 people in the vaccinated group had unprotected sex with partners with AIDS, while similarly 74 people in the placebo group had unprotected sex with AIDS carriers?. No need and no effect from the vaccine?. The only workaround is that they actually went out and proved that majority in both samples had unprotected sex with AIDS carriers. Otherwise, these results are bogus. I am not really sure how the results were computed. Am I missing something?. Anyone?.