Q: Some of my chickens will be vaccinated for Marek's Disease, but others are not vaccinated. Can my unvaccinated chickens get Marek's from my chickens who did get vaccinations ?
A: The Marek's Disease vaccination is attenuated, which means, in most cases, it's not even developed using the Marek's virus. Instead, it's developed using a different illness common to turkeys, but which causes the chickens in question to develop immunity to Marek's. This is the same way the small pox vaccine was developed for humans: that vaccine is actually based on cowpox. However, cowpox provided immunity from deadly smallpox.
We know of no reports or studies showing that nonvaccinated birds can be infected with Marek's Disease by exposure to vaccinated birds. Similarly, infants that haven't been vaccinated for Measles, Mumps or Rubella, yet, don't get those illnesses by being exposed to their mother who has had the vaccinations (or even when drinking her milk).