Somebody do the reasearch for me please. Maybe I'm wrong. I looked up info weeks back. Here's how I see it:
The virus is like a huge tank of piranhas. All the piranhas have sharp teeth and they'll take a chunk out of your finger if you put your hand in the tank. But a few of the piranhas are more viscious, more hardy and not only that, they have a nerve poison. You get bit by one of them, you not only loose a chunk of your finger, your gross motor functions are down until you recover (if you recover before you get eaten alive). OK fine, you stick your hand in the tank, you probably won't get bit by one of the few paralysis fish. The tank ecosystem is stable and there is no reason for the paralysis inducing piranhas to be more successful than the much more common normal piranhas. But you put a poison on your hand that kills the normal piranha if he bites you. One bite he's dead, you don't suffer so much of an injury. Pretty soon the population of normal piranhas decreases because they are dying off from biting poison fingers. Can't survive, they bite a poisoned finger and they're history. Now you have so few piranhas in the tank, you probably won't get bit at all if you stick your hand on there. But, the paralysis piranhas now have had there competition for biting fingers dramatically reduced. If you do get bit, it will probably be a paralysis piranha. The paralysis piranhas are biting fingers as much as they can and they're immune to the poison. They thrive because the normal piranhas are mostly all gone now and they're almost the only ones left to eat and reproduce. After a while most of the piranhas in the tank are paralysis piranhas that are resistant to the piranha poison and they're extra aggressive. All in all, the use of the poison temporarily made us safer from getting bit but ultimately lead to us being a lot less safe than we were if we just stuck our hand in the tank and accepted probably getting bit by a normal piranha. It's natural selection. It happens extremely rapidly in the microbial world because mutation is something viruses do all the time regardless of the environment they're in. However, if you make the environment less favorable for less lethal viruses, that only increases that chances of survival and success of the more lethal viruses. Does a virus have to be more lethal if it is more resistant to a vaccine? No but it often is more lethal if it also is more infectious.
This still leaves the question of how polio and other viruses have been mitigated by the use of vaccines if viruses mutate to become more infectious and cause more lethal symptoms.
