They were vaccinated for coccidiosis. Will the medicated food counteract the treatment?
Yes, it will cancel out the vaccine.
https://blog.meyerhatchery.com/2018/10/coccidia-prevention-vaccine-versus-medicated-feed/ : " The coccidia vaccines that are available in the US are “attenuated”, meaning they are a modified-live vaccine that could actually cause a mild illness in young chicks. Feeding medicated feed to chicks that are also vaccinated for coccidia will negate the vaccine "