> worrying that a human with a horse's head might be born anytime soon. But is there any advantage for a human to have a horse's head? I don't think. So it app

> worrying that a human with a horse's head might be born anytime soon. But is there any advantage for a human to have a horse's head? I don't think. So it appears to be a very flawed analogy, on multiple levels. Also you should consider that this proof reading mechanism is not a hard blocker for evolution, and it could be easily overridden by mutation that are highly favourable for the organism that posses it. And the ability to evade vaccine immunity is specifically such a mutation, in a population which is immunised with non-sterilising vaccine.