Why an AIDS Vaccine Has Been So Difficult to Develop

http://www.sciencedaily.com/releases/2012/09/120909150350.htm

ScienceDaily (Sep. 9, 2012) — For decades, a successful HIV vaccine has been the Holy Grail for researchers around the globe. Yet despite years of research and millions of dollars of investment, that goal has still yet to be achieved. Recent research by Oregon Health & Science University scientists explains a decades-old mystery as to why slightly weakened versions of the monkey AIDS virus were able to prevent subsequent infection with the fully virulent strain, but were too risky for human use, and why severely compromised or completely inactivated versions of the virus were not effective at all.

The research was conducted at OHSU's Vaccine and Gene Therapy Institute and is published online in the journal Nature Medicine.
Traditionally, there have been two methods for creating vaccines to combat infectious disease. The first approach utilizes a live, yet weakened strain of the disease in question. This weakened strain is not strong enough to cause illness yet potent enough to activate the immune system so that it can detect and fight a disease if it enters the body in the future. The second approach makes use of a dead form of the disease. As with the other approach, the introduction of the disease in a safe form educates and prepares the body for a possible future invasion.

In the early 1990s, a slightly weakened version of SIV, the monkey counterpart to HIV, was shown to protect monkeys for infection with the fully virulent version, but this weakened version was still able to cause AIDS in some monkeys and the protection was lost if the vaccine virus was further weakened.