But they're not 100% safe, and if the guards already have immunity, then they're not effective.

In fact, I may be reading too much into your dropping of the article on "vaccines" but there is a huge flaw in your logic. General facts about "vaccines" don't say something about a particular vaccine, especially a new one. But my apologies if you meant to write "these vaccines."

That's not a logical flaw; the logic is just based on an incorrect axiom.