I'm not sure. I think the vaccine is weakened Cocci bugs. The chicks need a couple of weeks exposure to develop immunity. The Amprolium doesn't let those weakened bugs reproduce enough for constant exposure so the chicks are not exposed to them long enough to develop immunity. But that's...
What were they vaccinated for? Marek's, Coccidiosis, or something else? The answer will be different depending on what the vaccine was.
Since you mentioned "medicated feed" I'll assume it is Coccidiosis. If I'm wrong, please let me know. There is no medicated feed for Marek's and if the...