You have antibodies that will bind to just about anything foreign, including viruses that your body has never seen before, M-type immunoglobulin (IgM). It's the kind of antibodies that responds in the first phases of an infection.

The problem is that IgMs are not very specific. They bind to a lot of things, but they bind poorly. In the library of Alexandria analogy, it's like a book that happens to mention a subject you care about, but only in passing.

Vaccines (and infections) result in your immune system developing IgG. That's the hyper-specific books for the pathogen you care about.

So it's not that the book wasn't at the front. Your immune system has to write the book on the fly, during the fight, using chopped up pieces of pathogen gathered by antigen-presenting cells.