Breeders/hatcheries do NOT regularly vaccinate against cocci. Cocci is what the medicated feed is targeted for.
It's all about timing. Read lazygardener's post above. If you're giving them medicated feed and they're in the brooder, great. But when they really need it is when they hit the dirt. Before that, they're not really exposed.
Since you live in Arkansas, I'm thinking you're warm and wet, which does increase your chances of a big cocci load in your soil. Warm and wet is what the little buggers love.
You can either continue with the medicated feed for a few weeks after they're on the dirt, or take them off and watch them closely. First signs are ruffled feathers, hunched over, lethargic. Bloody poops come a day or so later and by then you may be losing chicks. Be sure you have access to Corid, which is the medication used to treat cocci outbreaks. You'll need to get it to them ASAP after you notice symptoms, waiting for 3 day shipping will get you dead chicks.
You can also start exposing them now, to help build immunity. Take some soil from the yard and put it in the brooder. Gives them something to do, they learn to scratch, etc, and exposes them to a light load of whatever you have in your soil.