Izzybella: Do you have wild turkeys coming through your yard? They carry a less lethal strain of Mareks that IMO affords some good protection. (they catch and recover from this strain, most often without showing any signs of disease.) From my reading, the vaccine does not prevent infection of the bird, but it prevents the lethal tumors associated with the disease. So, inoculating your flock may prevent the disease from showing signs, but will not prevent the disease from entering your flock.