The circumstance of dying from a disease due to biological weakness has a suppressive affect on phenotypes which don't provide an immune system capable of fighting the disease. This is basic.
I am not making the argument that we are destroying species immunity to any particular disease by getting vaccines, which is clearly false. When you get vaccines or get a disease, the result is some degree of immunity to that specific disease (or some degree of immunity or maiming or death, in the case of getting disease).
Rather, bolstering the immune system with vaccines is a crutch that, while it may be the best option for an individual to choose, does still permit phenotypes which cannot handle the disease to be passed to offspring.
Obviously, this is a slow process. But also, just as obviously, someone who chooses not to vaccinate and thereby dies from a disease would have, otherwise, potentially passed that weakness on to their children.
this isn't bullshit or a niche theory. It's basic evolution, but in an area that is hard for some people to accept because they don't like it, and want to distance themselves from death, and feel like they are outside of the realm of natural necessity, or because they just can't conceive of biological robustness being that important, or being truly subject to any kind of degradation. But that is a failing to see the scope of the necessity to sustain our genetic robustness - not enforced by some creepy nazi idea is what's "perfect", through eugenics, but through sovereign choice.
And yes, making vaccines available does benefit the species as a whole, because we increase the ways we can fight disease. But those who fight a disease naturally, and actual actually accept the consequences of that, are exercising their individual rights in a way that is also beneficial to the species, by reducing the instances of problematic phenotypes, and (hopefully) breeding if they survive.