Don't know if this helps you but my understanding is that a lot of adult chickens have been exposed to some form of Marek's. There's different strains and some are deadly, but a lot aren't. That's why Marek's is given to newly hatched chicks rather then adults. You're getting the vaccine into them before they're exposed to another strain. Chickens can be given the vaccine at any age...but it only works if it's the first strain they're exposed to. That said, when I had my outbreak my vet had me vaccinate every bird on my property regardless of age. No idea if that helped or if the disease ran it's course, but deaths ended quickly after. I was told to let my current birds die out, disinfect and wait preferably a year before getting more birds. Then get vaccinated chicks from a hatchery. The idea was to rid my property of the disease. I do believe that it worked because though I vaccinate now, I don't vaccinate every chick...I vaccinate when both incubators are full and hatch at once and then I vaccinate the day old chicks and any other unvaccinated bird regardless of age. It's an annual affair rather then every new hatched chick. A leg band tells me if the birds been vaccinated and the bands color tells me if it was vaccinated as a new hatch or older. Orthodontic bands make great chick bands...you have to change them as the bird grows though with the permanent band after pol. I've never had a second outbreak so I believe my property is 100% free of that deadly strain of Marek's.
Ive found different vets give very different advice. It's like when my one horse had cancer. One vet said it was worth doing surgery. The same vet that gave me the chicken advice said yes, we can do surgery but he'd recommend against it because it was unlikely to be successful. When my kids dog had his eye punctured right before we got him, one vet said he needed his eye removed. My favorite vet said it's fixable. He sewed Billy's eye shut, intensive eye drop regimen and Billy's eye was saved. Billy wasn't ours at that point or I would have had the eye removed actually...but I've never known this vet to be wrong. I trust him.
That said, I have no idea what another vet would recommend or if the way I use the Marek's vaccine is considered acceptable. This vets an old old guy and often gives different advice from the other vets I use. I just know I've never had a second round of Marek's on my property and it's been years and years now.
Any way... thought I'd share the details of how I vaccinate in case that's helpful.
. This is the best picture I could get of the Zebra though…it was WAY to cold that day to walk out to where it was to get a better picture.
I have found that their fear response is biting. My bobtail bit me the first day I had her too, and my favorite Cochin girl does too 
They’re just such sassy hens!!
I’m honestly still learning about how the coloring works. 
