What breed are you hatching? some breeds are more resilient to it, silkies are not! I've had it in my patch, the adult birds were fine, but 2 of my young 4 month old silkie crosses came down with it and ended up being culled, 2 other birds from the same hatch were immune and were fine, so it all depends really it is the luck of the draw as to whether birds come down with it or not, the older they get the less likely they'll go down with it, I believe it is been spread by wild birds? if you have never had a case of it you may be lucky and not have it, though if your birds have been vaccinated they may now be carriers and could shed it? it's a tough call, if it was me I wouldn't vaccinate and would rely on the birds natural ability to fight it, I don't know anywhere near here that has the available vaccine either but I will never raise silkies again knowing that Mareks is out ther.