I vaccinate all of my chicks for Markes Disease. I just sold 6 one day old chicks to someone who will put them under a broody hen. Is it possible for his hens to get the disease from vaccinnated chicks?
The vaccination makes no difference as to whether a chicken can transmit Marek's. Chicken Marek's vaccination is actually Turkey Marek's. The vaccination does not prevent chicken Marek's. It prevents the lesions from forming that does the damage. It is possible for chickens that have been vaccinated with turkey Marek's to catch chicken Marek's and to transmit chicken Marek's. I am pretty sure they will not transmit turkey Marek's to other chickens right after vaccination, but I'm not 100% sure about that, only about 99.9% sure.
If your flock has chicken Marek's and your chicks were exposed to the flock, it is possible that they are infected with chicken Marek's and will give it to the other hens. If your flock does not have chicken Marek's, no they will not transmit chicken Marek's due to the vaccination. If you hatched yours in an incubator and they were never exposed to the flock, they will not transmit chicken Marek's to the other chickens because of the vaccination.
I haven't been able to find any definitive answer to this in the research I have on hand. There is, however, a very good article on Mareks written by Dr. Peter J. Brown at firststatevetsupply.com and in it he invites questions on the topic by calling: Peter Brown at 1-800-950-8387. Alternatively, you could try calling your State Veteriarian for his/her opinion on this.
My best guess is that the answer will be something like, "it's not impossible, but it's not likely, either." The Mareks vaccine does not prevent the disease, per se. I believe it is what's called a "live attenuated vaccine". These types of vaccines are usually derived from the naturally occurring germ. They can infect, but not cause serious disease, and encourages the body to create an antibody response to neutralize it. What Mareks vaccine is supposed to do, then, is prevent the cancer-like cells (T-cells) from forming and producing the lesions on the brain or the visceral tumors that impact vital organ function.
Hopefully, one of the experts will get back to you quickly with an answer to this question. My layman's opinion is that it makes sense to vaccinate not only the broody hen, but the entire flock with the Mareks vaccine. Even though the risk of Mareks infection declines with age (they call it "age resistance"), there has been some research showing that vaccinating older birds, even those already infected, does have value.