Been a while since I've dealt with marek's desease.
There seems to be some variance in susceptibility to marek's, Some breeds, sometimes even family lines within a breed seem to have better natural resistance. Unfortunately it always seemed to be a youngster that I had high hopes for as a future breeder that would come up limping. Before I started inoculating my Brahmas as day olds, I might expect to lose somewhere between 5 to 10 percent of that years hatch on an average year.. Since I've been inoculating the past 12 years or so, have had no problems.
Now your mileage may vary, depending on how resistant your birds are.