I have to voice my opinion on the Merek's vaccine. It would be great if it worked to completely prevent Marek's but unfortunately, it doesn't. From what I've read, all the vaccine does is prevent the symptoms of it. So you could have a flock, full of merek's disease, spreading it and passing it on with absolutely no knowledge of it being there. I'll have to see if I can find the article about it. I personally prefer not to use the vaccine. That way, if there was ever a problem, I would se it immediately.
There is a lot of truth in that and it is my philosophy also. Some breeds have proven very hardy on my farm, others more fragile. I stick with the ones that do well and that I like. The Black and Silver Ameraucanas and Cream Legbars have been very satisfactory, whereas the Welbars, Welsummers, and all forms of the Lavender Ameraucanas are less so. I'm still working with those (except no more Welsummers) and am actively selecting for hardier genes, along with crossing back to stronger varieties (lavender to black is very promising).
I haven't tested for Marek's, but as I sell almost all as chicks and Marek's is not passed from hen to chick via the eggs, my chicks are sold "clean" in any case. I like to think they are healthy and sturdy enough to survive a future exposure to any of the common diseases.
According to Merck's Veterinary Manual, Marek's is so common that one should assume every chicken over a few weeks is infected. The fact that so few ever show symptoms tells me that in a low-stress environment, most of our modern varieties have great immunity without a vaccine.
I read a research paper a while ago that suggested the industry's use of Marek's vaccines may be allowing the survival of very pathogenic strains of Marek's. A virus must tread a thin line and not be too deadly or it will die out. Good examples are HIV (never kills directly, has spread over the world) and Ebola (very high mortality, dies out in humans after each epidemic, survives in bats where it is not very deadly). The put unvaccinated adult birds with vaccinated ones and found a higher incidence of mortality from Marek's than expected, meaning the healthy looking vaccinated birds were carrying a more lethal strain of Marek's.
Because of this, I would never let a vaccinated "spent hen", anywhere near my birds, and I'm leery of vaccinated older birds in general, though vaccinated chicks would not come with a bad strain unless they were exposed to it in the hatchery (which would kill all the non-vaccinated chicks and put the hatchery out of business).
