In 15 years of keeping chickens, I've never lost a bird to coccidiosis. (I lost one new chick to pasty butt despite efforts to treat it). I have no idea if the chicks I buy have been vaccinated. I always just buy medicated Start & Grow feed. I clean the brooder often to keep them out of their droppings. When they're old enough to spend time outside, they are gradually exposed to organisms in the ground, which I believe helps boost immunity. I've probably never left them on starter/grower for the recommended 18 weeks. It's too hard to keep the different ages out of each other's food after they integrate into the flock. My chickens don't live on processed feed alone, anyway. They get a lot of their nutrition from free ranging and kitchen scraps. Now, when I had ducks and geese, I definitely fed non-medicated starter as recommended.