I was reading a couple of those reports regarding the way they checked for Mareks, and developed what they're calling a vaccine for it. While a great deal of it was over my head, there were a couple places in each of the reports that talked about how some of the newly hatched chicks gave a positive results, but it was simply antigen, not an active case. It mentioned that the antigen was produced, either in the yolk, or immediately when they hatched. This is part of why the vaccine has to be administered within so many hours of them hatching.
What I deduced from that, if a percentage of newly hatched chick shows antigens immediately after hatching, but the pharmaceutical company didn't consider it to be an active case, then there is an immuno-response in a certain percentage of first generation chicks. Of course, those chicks were disposed of, so they could do their research. The pharmaceutical company was not interested in learning how much resistance those chicks would have had naturally. Nor were they interested in letting those chicks grow up, become exposed, and see if the next generation of offspring had even more resistance. If they did, they did not publish their findings, because it would have been counter-productive to their goal, of making, and selling a vaccine.
I still think those with flocks that have Mareks, could easily close their flocks (no new birds coming in, or going out) hatching from your own eggs, and within a few generations, have a flock mostly, to totally resistant to the strain of Mareks affecting them.