The vaccine prevents most chickens from developing symptoms and dying from mareks disease but since it’s a leaky vaccine, it doesn’t prevent the chickens from actually contracting mareks and infecting other chickens. So if you bring home a chicken that’s vaccinated for mareks but carrying the disease, if you have a flock with unvaccinated birds and introduce that vaccinated bird carrying the disease, they’ll spread it to the unvaccinated birds and they’ll get sick and show symptoms meanwhile that one vaccinated chicken won’t due to the vaccine preventing symptoms from showing. That vaccinated bird with mareks disease is now spreading the disease all over the property and will spread it to any new chickens, vaccinated or not. I do see that they have strict bio security measures with their hatchlings that they sell, and you can prevent them from contracting the disease from the main flock with a strict bio security regime, but I honestly would not risk purchasing from them because I wouldn’t want the chicks that had a bio security breach like some of the reviewers, and end up with mareks on my property that won’t go away for years in the soil.

By that point you might as well get ducks for a few years since they do not get mareks disease!
Here’s the review where the owner told her some of the hatchlings were exposed to the disease that’s in her main breeding flock.
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