To be effective MD vaccine needs to be given behind the neck when the chicks are less than 24 hours old and then the chicks need to be isolated with no exposure to other chickens for 10 days. This link shows a graphic demo of how to vaccinate for MD...(warning may be disturbing to some!
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http://poultryone.com/articles/mareks.html

http://poultryone.com/articles/mareks.html