The recent RV144 vaccine trail showed a 31% reduction of HIV infections among trial participants over three years. That's good -- better than any previous trial results -- but still not good enough.
Now a paper published in Nature and presented at the AIDS Vaccine 2012 conference in Boston sheds light on why the combination of vaccines used was not more broadly effective. It had already been determined that the trail participants who did not contract HIV were producing antibodies against the V1 and V2 loops of HIV's protein shell. Genetic analysis of the virus in those who did contract HIV now shows that they were infected by strains that had mutations in the V2 loop.
It's not a home run, but for the first time we know why an HIV vaccine worked for some people but not others.