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.
You know... this is the first time I saw the tacked on "Donald Trump" they didn't even manage to match the font of the original name. They somehow did a worse job then "The Ichy & Scratchy & Poochy show" did in the Simpsons.