This is the response from the vet when I asked if I should vaccinate chicks hatched by a broody.
If the chicks are raised by a broody hen, then the risk of Mareks can be higher even after vaccination. If the chicks become infected with the virus before the vaccine takes effect, they will develop the disease. To find out whether your hen carries the virus is expensive (It is a PCR blood test - about $100 plus shipping).
I already know that my coop and brooder have Marek's in them, so there is no point having my hens tested. The chicks are all going to inhale Marek's at some point, it is just a matter of when and how much. I'd love to be able to hatch with a broody again. I just want to know if there is any point vaccinating the chicks when they hatch.
I lost two pullets already that were hatched here last year. One was hatched by a broody from eggs I purchased. She became sick at a very young age but lingered for a long time before succumbing to the disease. The second was a very young pullet that hatched in the house from eggs that came from my flock. The chicks were raised in the house, were not vaccinated, and didn't have exposure to the other birds until they were 6 weeks old. Unfortunately I gave away half of the chicks before I realized they were exposed to Marek's. Out of 16 babies, only the one pullet has shown any signs of illness (thank goodness). I do think the broody raised chicks were sicklier than the ones hatched in the incubator. However, the broody hatched eggs were not from my flock, so there would be no "natural resistance" compared to if they had been my own eggs.
Hopefully this all makes some sense. I think I will let my broody raise chicks. It will be extremely sad if they get Marek's and have to be put down, but if I think of it as "helping science" it won't all be pointless.