(long winded vaccine explanation. feel free to skip lol)
The mareks vaccine is 'leaky'. What it does is prevent the bird that was vaccinated from dying (more or less, I'm sure it's not fullproof), but they are still contagious. So while an unvaccinated bird would get mareks, get sick, youd notice and quarentine it from the flock.... a vaccinated bird would get mareks, not get sick, and run around and give it to way more of your other birds than if it had quietly deteriorated in the corner and been removed. So if ALL of your birds are vaccinated, they could all get it and not get sick, and all is fine, but if some have it and some dont, the vaccinated ones are much more likely to give it to the unvaccinated ones. Granted, if you have like 4 chickens in a tiny coop, both situations might give everyone mareks, but I have a ton of birds on a ton of land, so I opted against the vaccine.