Why an HIV vaccine has eluded scientists:
The virus is the most diverse we know of. It mutates so rapidly that people might carry millions of different versions of it, just months after becoming infected. HIV’s constantly changing form makes it unlike any viral foe we have tried to thwart with a vaccine.
I've always been a skeptic of a vaccine for these reasons, especially when treatment can acts as an effective vaccine, by sharply cutting viral loads and thereby infectiousness. The FDA may soon approve an anti-viral, Truvada, for HIV-negative men who want to stay that way. But the goal of a vaccine is not entirely quixotic, given the pace of scientific advance:
It took 47 years to create a vaccine for polio after the microbe behind it was identified. The measles vaccine took 42 years. The hepatitis B vaccine was a positive sprint at 16 years. “Twenty-eight years isn’t an inordinate amount of time,” says [Anthony Fauci, an immunologist who heads NIAID].
Alice Park has more on one of the leading candidates for an effective vaccine, developed in 2009.