Had those mice been immunized against influenza? That's not what I read, but I might have miss it. What I read is that they used the vector without any payload, and it turned out that influenza is an example of something nominally unrelated where resistance was slightly increased while candida albicans (no idea what that is, not a native speaker) are an example for slightly lessened resistance.

Lines up well with my mental model of how the immune system deals with the unknown: a million monkeys not on typewriters but on text editors with copy/paste. Who fire new code, aimlessly mashed up from their library of previous experience, at a test fixture. Any change to unrelated previous experience can increase or decrease their chance of finding something that sticks.