Last year I asked a poultry vet at MSU about this and other things regarding Marek's. Here are copies of his e-mailed responses.
Dear Jody:
Unlike some diseases of poultry, that is not true for Marek's disease. The vaccine is supposed to prevent infection rather than just mask the symptoms. The plan is to have the birds infected with the Marek's disease vaccine virus before they get infected with the "field virus."
Hope this helps,
R. M. Fulton, D.V.M., Ph.D.
Dear Jody:
I understand your and your poultry pal's concerns but you are right in your thinking about Marek's disease. Marek's disease virus is
everywhere. In fact, the vaccination of poultry for Marek's disease is a race to see which virus is going to win in the bird.....the vaccine
virus or the disease causing virus. While it is true that Marek's
disease vaccinated birds are infected with the vaccine virus for life, I am not aware of them shedding the vaccine virus. The good news is that the vaccine viruses do not cause disease so your chickens are safe and not "Typhoid Marys." The other vaccines used in poultry do not cause "Typhoid Marys" except for almost all vaccines used to protect against Infectious Laryngotracheitis. Chickens vaccinated with those vaccines can shed the vaccine virus and the vaccine virus will make non-vaccinated chickens sick.
As for vaccinating your chickens for other diseases, I only recommend vaccinating chickens against diseases you have had or that are in your area. In most cases, that only includes fowl pox. Please note that chicken pox is a human disease and not a chicken disease. The virus that causes chicken pox in humans does not cause disease in chickens nor do humans get it from chickens.
Hope this helps,
R. M. Fulton, D.V.M., Ph.D.
I also spoke to a vet a Marek's vaccine manufacturer and received similar info.