The difficulty I see here is that it took the yeast not very long to mutate a beneficial effect, but it could just as quickly mutate away from that and even mutate something harmful.

That seems unlikely. The entire way it works is the yeast produces proteins that "look" like a virus to the immune system, causing a immune response. However that's the only part of of the virus in the yeast, there's nothing else that completes the virus. It's like... a steering wheel without a car, no matter how hard I mime swerving to run over a pedestrian nothing's gonna happen, because there is no car.

If the viral protein mutated it'd either A) still be recognized as a viral protein and generate immunity for a virus that doesn't exist, or B) generate a non-functional protein that the body would simply digest.

[–] squaresinger@lemmy.world 1 points 3 hours ago

The version A) is a big issue actually, thanks to the Hoskins effect.

That means that if the immune system is trained for a slightly wrong type of pathogen, it might have a worse immune response to the actual pathogen at hand than if it wasn't trained at all.

So if that viral protein would mutate a bit, so that it's still recognized as viral protein by the immune system, it might cause the immune response to the actual virus to be worse instead of improving it.