Let’s start with what a vaccine does.
Quick and dirty high level overview, handwaving over the technical details: A vaccine trains your immune system. A vaccine introduces your immune system to a pathogen, or a fragment of a pathogen, and says “if you see this again, destroy it.”
Your immune system is very, very slow to respond the first time it sees a virus, but it remembers. The second time it sees a virus, it can respond quickly.
Think of a vaccine as a “Wanted” poster. You can show the police a picture of a criminal, so if the police see the criminal they can arrest him right away. They don’t have to wait until they catch him in the act and then arrest him.
So a vaccine builds up your natural immunity to a virus. It says ”Hey, remember this. If you see this, it’s bad.”
Okay, so what does HIV do?
HIV destroys the immune system.
What does a vaccine do?
A vaccine teaches the immune system how to respond.
If your immune system is destroyed, it can’t respond.
Imagine that a vaccine is like a “Wanted” poster. The poster tells the police “if you see this guy, arrest him.”
Then the guy rolls into town and machine-guns all the cops.
It doesn’t do a lot of good to show the cops a “Wanted” poster if all the cops end up dead. If you kill all the cops, there’s nobody left to make an arrest. If you destroy the immune system, there’s nothing left to respond to the virus.
So why doesn’t a vaccine react the immune system to recognize the virus and act really fast so it can wipe out the virus before the virus wipes it out?
We don’t know. It seems that even if you try to teach the immune system to recognize HIV, it still has a lot of trouble responding to the virus. To use a really, really crude analogy, it’s like there are no good photos for “Wanted” posters—they’re all so blurry the cops are like “Who is this? I can’t tell who this is.”
But in general, it’s hard to teach your immune system how to recognize a virus when the virus targets and destroys the immune system.