Others have said it and I will, too. Mareks vaccine does not prevent the disease, only the tumors associated with it. As for me, I have never vaccinated any chicks and never will. Mareks has never shown up here in the dozen years I've had my own flocks, but I never buy started birds, never trade birds, not even chicks. All are hatched here or came from direct hatchery shipments to local feed stores that had brooders (though that was years ago. Now I have breeder quality birds since they rarely develop reproductive malfunctions and live a whole lot longer and lay longer). I also practice biosecurity and there are no rats/mice in my barn, no wild birds nesting in there, etc.
It's up to you, but you just need to have all your facts to make your decision.
It's up to you, but you just need to have all your facts to make your decision.