Try this, Type in "should chicks be vaccinated" Its an old topic from this past November... on the 4th comment down, it talks about protecting them organtically.
I've always heard that by giving them medicated food, it voids the vaccine. I heard it here last year..I wondered how that could be too... Try typing in the coccidal vaccine in the search area and see what comes up, I know theres been several dicussions about it. I hope that helps.
it's not serious, it just voids the vaccine they got at the hatchery, keep them on the medicated food now. You'll still be able to eat the eggs and meat. Don't sweat it.