Welcome! There's some confusion here'll try to simplify the story.
Chicks can be vaccinated against Marek's disease, which is a virus, and then be isolated from exposure for two or three weeks so they can develop immunity.
Most medicated feed has amprolium, which helps chicks manage their possible exposure to coccidia, which is an intestinal parasite found in soil. Some of us are blessed to have minor or no issues with this parasite, and can safely not feed medicated starter. Other places, and they can be in the same neighborhood, have huge issues, and chicks need the medicated starter to avoid illness and death from a too rapid 'invasion' of this parasite.
There is a 'vaccine' against illness from coccidia, and it's a low dose of the parasite given to day-old chicks. If they then are fed amprolium, this 'vaccine' is wasted.
As new chick owners, the safest plan is to feed medicated (amprolium) feed for eight to twelve weeks, until after the chicks have been on your ground for a while, because you won't know if your site has a problem or not.
Mary