>AGSI we're suspending disbelief, with ayy lmaos space-aliens from the galactic federation, or whatever pleases you, and ham-fisting a 100% lethal virus into existence. What can be done to stop it?

There are organisms that have effective immortality, the first thing to check is if your 100% lethal virus works to effectively kill tardigrades. If it doesn't, then you could figure out how they're surviving, and then make a copy of that system in humans as an additional hedge of protection inside the human immune system.
https://www.youtube.com/watch?v=zV2wIfDzFMk