I've just read this thread with great interest and am shellshocked about the misinformation that is out there.
Firstly - chickens vaccinated against Mareks disease are carriers AND shedders for life. Here is just one source:
http://www.ncbi.nlm.nih.gov/pubmed/15757475
I learnt this the hard way.
The season before last I lost two thirds of my flock of heritage breed birds to Mareks disease. The cause? I had bought 6 day-old Hyline pullets which had been vaccinated against Mareks. The strain of virus that had been used to vaccinate these chicks had only recently been introduced. My heritage breed poultry had no immunity against this strain of Mareks disease.
None of my birds who were adults when these commercial hybrids joined our flock got Mareks. This tells me that vaccinating an adult bird is not necessary.
None of my chicks who were older than 6 months at the time the vaccinated chicks arrived succumbed to Mareks. So their immune system must have been strong enough to develop immunity in time.
I raised the Hyline pullets in the same brooder as 3 other breeds: Silkies, Orpingtons and Cochins.
None of the vaccinated Hyline chicks died. All grew up healthy and strong and a few of my free rangers were able to walk right up to their brooder which is in the garage.
All Silkie chicks died from Mareks - not only those who grew up with the Hylines but also all that were hatched long after the Hylines had started to lay. The parents were unaffected.
About half of my Orpington and Cochin chicks died. This included those who were raised with the Hylines and all chicks hatched and raised while we had the Hylines. The survivors were those who were hatched and raised by a mother hen (didn't help the Silkies, though). Some hen hatched and raised chicks still succumbed to Mareks but the incidence was a lot lower than among incubator hatched chicks.
Death by Mareks occurred between 3 weeks and 18 months of age. I have done post mortems on many of them and most had tiny tumors all through their lymph system. Only about one third displayed classic Mareks symptoms like paralysis. Others were emaciated or blind or had huge skin tumors.
What they all had in common was that they were bright and eating well until 2 or 3 days before they died (or were euthanised).
This past season I lost maybe half a dozen chicks to Mareks - out of hundreds. I do not vaccinate.
The most important thing to do is to slowly and carefully subject new chicks to the virus. This can be done by either letting a hen raise them or by using common sense when caring for chicks in a brooder.
I don't sterilise my brooders. I clean them with water (they are made from plastic), let them dry in the sun and fill them with new bedding material. Drinkers and feeders are scrubbed with water and dishwashing liquid.
I don't sterilize my clothing or hands or anything when making the rounds.
And last but not least - I guess those individuals who were not strong didn't survive and therefore weren't able to pass on their weak genes which resulted in far fewer losses. I do no longer have Silkies.