It’s my understanding that some vaccines for viruses are made by injecting a person with a very weak form of the virus, which their immune system is strong enough to kill off. Then, when the real virus invades, antibodies are already present, and the infection is quickly killed.
So, why can’t this be done for HIV? I have never heard this explained in the 25 years since it surfaced why it has been so difficult to develop a vaccine.
The first vaccine for smallpox wasn’t using weak smallpox. Instead, they used cowpox.(Sorry, no cite, I’m remembering this from a project I did 5 years ago.) I don’t know if they still did this in this century, however.
Something that was mentioned in my college biology class was that HIV attacks the immune system itself, which makes it a bit more complicated.
HIV is a special type of virus known as a retrovirus. IANAPathologist, but I do know that no one has ever invented a vaccine for a retrovirus yet.
A bit about retroviruses, written for the layperson:
http://www.ama-assn.org/special/hiv/newsline/briefing/retro.htm
Smallpox was a lucky shot in the dark. Some viruses mutate at a ferocious rate, making vaccines impossible. We’re fortunate in that only one of our most dangerous diseases mutates that fast; the cold has been impossible, the flu somewhat less so, but only AIDS has been both deadly and rapidly-mutating.
Mainly because it’s caused by dozens of different viruses and it infects mucosal surfaces, where you get very little long-lasting immune memory.
But, yeah, you’re right about why HIV has been such a problem.
I heard AIDS vaccine clinical trials were successful so far, so a vaccine may not be too far off. But the virus mutates so rapidly - there can be up to 4 different variants in one person alone - I can’t see how a vaccine would be completely protective. Better than nothing though, that’s for sure.
As I undertand it, the reason why HIV mutates so fast is because it is a retrovirus. Now, I am no geneticist, so the following is pure speculation, probably misinformed.
When HIV enters a cell, it uses an enzyme called (I think) reverse DNA polymerase inorder to synthesise a complementary DNA strand to the viral RNA strand. This DNA strand may be spliced into the cells genome, and thereafter used to synthesise both the proteins utilised in the protein coat of the virus, and the RNA which is the ‘blueprint’ of the virus.
I’m just guessing here, but I would say that whereas DNA mechanisms are fairly efficient, the same could not be said for RNA repair mechanisms, if they exist.
The reason why I should think this is so is because if there is a flaw in a single mRNA molecule, there is no problem, the cell might produce one dud protein molecule, but that is about the extent of it. A change to DNA is much more serious, as this means a dud gene will keep producing dud proteins.
The nature of the enzyme used to synthesise a complementary DNA strand from the viral RNA might also promote an increased mutation rate. The mutations that really matter in terms of developing a vaccine are the ones changing surface proteins, or antigens, as hese are what are targeted by vaccines, which stimulate the production of antibodies to the antigens contained in the vaccine.
As I said, all this is uninformed speculation, I have not even started HS Bio yet, I’ve just read the text.
-Oli
Wow Starman…im pretty sure i never used the words “reverse DNA polymerase” EVER in high school 
<last hijack i promise> and starman…if your reading your textbooks BEFORE your even taking the class in High School, do me a favor…go outside and play with your friends 
Keep up the good work, Starman. And you don’t have to do anyone else here any favors, you can do what you want.
QtM, MD
Who remembers all too well being held in contempt by certain peers in high school for not hiding my interest in intellectual pursuits well enough. Certainly one should not have to hide anything like that here at The Straight Dope, especially in GQ.