This is just what I read, I don't know it actually makes the vaccine better than a normal immune response, which I imagine is also robust.
There was recently (but pre-COVID) a problem with a vaccine (I think TDAP?) that was reformulated to use specific proteins instead of whole killed bacteria, and produced less persistent immunity as a result.
I'm sure the powers that be have their reasons for not going into detail, but I am interested as well. I'm sure I only understand a simplified version, but I thought that a vaccine worked pretty much by invoking an immune response. As a curious layman, I'd like it explained to me. Just to satisfy that curiosity. Web searches give me the "CDC says it's better" without going into the "why."
When you are infected with anything, you do not always develop anti-bodies. If you do develop anti-bodies, they might not be effective at actually stopping you from being re-infected.
Everything about the virus can mutate, but the virus will always target the ACE2 binding site. The spike protein can mutate to bind to it differently, but it will always require a spike protein. (Biology is Analog, not digital)
Thus targeting the spike protein is more efficient as it will be highly conserved.
There is a video series that does a nice job of breaking down how your immune response works: https://youtu.be/lXfEK8G8CUI
The vaccine is more effective at developing anti-bodies for the spike protein as it is the only protein exposed during the process.
An analogy that has helped some of my family: The virus is like a missile, and your immune system is a anti-missile system. Regardless of what the missile looks like, it has to have a warhead, and that warhead is what really defines what the entire thing does. So, if your defense systems targets the fuel of the missile, the enemy can change what fuel it runs on and evade your defense system. They can change the material it is made of, the shape of the overall device, change the propulsion and navigation systems…but the warhead can only have minor changes as it has because it it makes too many, it becomes ineffective and is no longer a threat.
Thus, if your defense system targets the warhead none of the other changes matter.
But the ACE2 receptor is relatively lenient in what it will pair with compared to the immune system.
So a spike with multiple mutations is more likely to be able to continue infecting cells via ACE2, than the mrna-vaccine-induced-immuno-response (targeting only the spike region) is able to block it.
And because of this "fuzzy" matching, less than perfect mutations can perpetuate until other helpful mutations occur.
But each of these mutations is also a step closer to evading detection by the immune system, so they can be helpful over multiple iterations of mutations, even if there's no selective pressure/benefit provided per mutation. That it can continue to bind to ACE2 is enough, until it reaches a point where the immune system doesn't recognize and must re-learn that target.