I'm saying that at most using the vaccine will cause a virus to evolve that it evades the vaccine. It won't get any other supervirus properties, become worse, or otherwise change. The concern with using antibiotics (for instance) is that a bacteria becoming immune to one becomes more resistant to all and it's hard to generate new antibiotics. On the other hand, with the vaccine, you just might need a booster with a different signature. There's no "broadening". It's just different.

So, a vaccine's immunity might not last forever. And?