With some brooder modifications, I raised chicks with a broody hen this summer. They were vaccinated at hatch and then put back with their mom. One of the pullets showed signs of Marek's at 10 weeks. She obviously was exposed to the virus at some point before she had built enough immunity from the vaccine. I was sure she would die, but decided to ride it out. She was sick for about 3 weeks before starting to improve. She actually made a full 100% recovery and is a normal pullet now. I totally expect her to have a relapse and die in the near future, but only time will tell.
I thought if they were vaccinated at hatch they needed to remain in a Marek's free environment for 3 weeks to develop immunity.
I have had Marek's in the flock for 2 years and most all losses were young birds that I brought in, the stress of integration seemed to trigger it.
This year I have had 3 broody raised clutches with NO Marek's symptoms in any of the chicks (knock on wood)! They are a variety of breeds so it can't be chocked up to genetics, 2 were homebred bantam chicks and the other 4 were purchased from a breeder at 1 day old (breeds included heritage RIRs, Norwegian Jaerhon and a pretty blue pullet who's breed I can't recall). They are 3 months, 4 months and 7 months and doing great.
None were vaccinated and all chicks were raised with the flock from the start so they never have any integration stress as juveniles, I believe that is the thing that has kept all of them healthy and Marek's free. Fortunately we also didn't have any minor injuries or predator stress to trigger a Marek's episode.
Here is a photo of the 7 month old bantam roo, his mom is a BB Red and his daddy is a Ginger. He may be a mutt but I think he is pure eye candy in a tiny package.