HIV has resisted attempts for a vaccine for three decades, and despite vastly improved ways to treat the condition, two million people still get infected every year. A new gene therapy technique has been shown to provide complete protection from HIV in mice with humanized immune system, and the same thing should work in humans, too.
The way gene therapy works is by using genetically modified viruses to deliver new DNA into cells. Once the DNA gets inside, it merges with the cell's own DNA, "reprogramming" it to do something new. In this case, CalTech researchers have introduced DNA into muscle cells, teaching those cells how to manufacture HIV antibodies. HIV is especially nasty because the immune system has trouble effectively targeting the virus, so even if you've got the right antibodies, there simply aren't enough of them. Once you've got your muscles pumping out antibodies, though, the HIV viruses get overwhelmed, and your body is able to protect you on its own.
Gene therapy is risky because there's no way to undo it: once your cells have been reprogrammed, you can't un-reprogram them, so if you turn out to be allergic to something that they start producing, you're in some serious trouble. At the same time, there are some major advantages: all it takes is one single intramuscular injection, and you get lifetime protection. And it's extremely effective: in mice with humanized immune systems, two different types of antibodies managed to be "completely protective" even when the mice were exposed to HIV doses that were a hundred times more concentrated that you'd expect to get through natural exposure.
With the success of the mouse trials, the CalTech researchers hope to transition this technique into human trials within the next few years. It's a tricky process, since mouse research doesn't always pan out in humans and we're going to have to be very careful to make sure that it's all safe, but from a technical perspective, we can definitely make this happen in the near future.