The Marek's vacccine is considered "leaky" because it stops them from developing the tumors associated with the disease, but it does NOT stop them from getting the disease. So vaccinated birds that are exposed to the virus can catch it, they can shed it, but the rate of symptoms and mortality will be much, much lower amongst them. So an unvaccinated bird may show symptoms and die, where a vaccinated bird may show not symptoms and not die, while both can still be infected with the disease. If you have never had a case of Marek's in your flock then you don't need to worry that at this point in time there is any exposure, to any of them. Bringing in baby chicks is very low risk of bringing in disease. Bringing in older birds comes with more risk and those birds should be quarantined for at least 4 weeks to give anything they may be carrying a chance to show symptoms before you expose the rest of your flock, that includes all kinds of illness. When birds free range they can be exposed by from wild birds. The Marek's virus can travel on the wind as much as 5 miles. Unless birds are kept in a hermetically sealed environment, there is no way to make the risk zero, the virus is very wide spread, but by being as careful as you can, obtain birds from sources you are very sure of, take basic precautions, you can lower the risk some. I have Marek's in my flock. I've not brought new birds in of any kind in 2 years, I've been hatching my own from the stock I have that are not showing symptoms. Many of us do that to try to breed birds that are more resistant to the virus. Unless your birds contract a very virulent strain of Marek's (some strains are much more virulent than others) then not all birds will show symptoms of it. In many cases it's managable, though you will have losses. But that only matters if you actually have a confirmed case. Until then, enjoy your birds, vaccinated or not.
