I started vaccinating for Marek's several years ago, after it practically decimated a shipment of pullets I'd ordered to resell. Prior to that, none of my birds had been vaccinated. I knew the vaccine is only recommended for day-olds, but I had plenty of leftover vaccine so I hit the adults, too - figured either a) they hadn't been exposed and it would protect them or b) they'd been exposed, were resistant, and it would do nothing. I had no problems with any of them, and I've had several chicks that never got the vaccine because they hatched under broodies. Knock on wood, no issues with them either. However, someone recently raised the question of introducing vaccinated birds into a flock of unvaccinated birds, since the vaccinated birds could still be carriers. So I guess there are 2 main questions that I could not answer:
Is it safe to mix vaccinated & unvaccinated birds, such as someone with an unvaccinated flock purchasing a single vaccinated bird, and if so, because it's a live vaccine, how long after vaccination should you wait before introducing them?
If a breeder who has not been vaccinating for Marek's wishes to begin doing so, how should they handle the older birds? I kind of assumed that, since I'd had an outbreak, anything still alive was likely resistant, but if someone is unsure if their birds have ever been exposed, they might want to proceed differently.
edited to add: I'm pretty sure this has been covered before, but we all know I'm search-challenged!