You never get chicks or chickens from outside to add to your flock. They all must be vaccinated at a day or 2 of life. Vaccinated chickens can still CARRY Marek's. The vaccine only prevents them from dying from it. It's taken me years but all my chickens are now vaccinated.
It is worth observing that if the vaccine "only prevents them from dying from it" and yet you still say that "contact with other chickens . . . could kill your flock," the vaccine is relatively ineffective.Do not allow any chickens into your flock that have had contact with other chickens. It's a risk. It could kill your flock.
I prefer the non-vaccine route. Scientists of today generally acknowledge that the Marek's vaccine has been the cause of its present degree of spread throughout the world.
[Link to National Geographic article on Marek's vaccine enhancing spread of the disease]
[Link to PubMed article on the Marek's vaccine increasing the virulence of the disease]