> Actually, the ability to respond to never-seen-before threats is the whole purpose of the adaptive immune system [1].

But the poster I was responding to was claiming that our immune system should have evolved the ability to not respond to those threats, but anticipate their specific forms without seeing them. That vaccination as a concept is hogwash because if it was useful, we'd have done it through evolution. But that's plainly nonsensical - we wouldn't need a system that functioned by random variation if we evolved the ability to know what the real targets would be, but we couldn't do that without specific knowledge, which we can't get without being exposed to the things, by which time ... it's too late to have then already built the specific defenses.

And during the time that takes you're suffering the consequences of a viral infection. Majormajor was replying to a comment suggesting that it might somehow have been possible to evolve the equivalent of a COVID-19 vaccine, which allows your immune system to adapt without experiencing a viral infection. What couldn't possibly work is for your body to already naturally be immune before experiencing the virus.