Vaccinattng for Mareks does not always eliminate the potential to get Mareks. I haven't vaccinated for anything here in 40 years and the birds here don't get Mareks. Maybe Mikes birds have a natural immunity......which is far better than an attempt to make them immune through vaccinating.
Walt
Since the chicks come from the same hatchery at both Concord Feed and Mike's, resistance in the chicks in one store and not in the other seems highly unlikely.
Yes, the immunization doesn't always prevent the disease; but not all immunizations take effect. In any event, the immunization is incredibly effective at preventing the associated tumors.
Not immunizing for Marek's is equivalent to not taking precautions against AIDS-HIV.