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Second question first: The Marek's vaccine is a live virus. It is weakened, but it is live. I don't believe there is a killed virus vaccine for Marek's, but I welcome the correction if anybody knows better.
First question: The vaccinated chicks will likely pass the virus to the unvaccinated chicks. From what I understand, they will be asymptomatic, just like the vaccinated ones, but they will actually carry the "lab" version of the virus (weakened). I am still researching what that means (coincidentally, I was just looking at this issue a few days ago), but from what I can tell so far, it isn't a big deal. In deciding whether to vaccinate the unvaccinated ones, the considerations are the same as they would be if you didn't have vaccinated chicks. Although the vaccinated chicks will likely pass the "lab" virus to your unvaccinated chicks, the unvaccinated chicks can still get the "field" virus. So, apparently, picking up the lab virus from the vaccinated chicks does not afford the same protection as being vaccinated. Even being vaccinated is not a guarantee against getting the field virus. In fact, the vaccine doesn't prevent infection with the field virus at all. It only reduces the likelihood of symptoms. So your unvaccinated chicks could get the field virus from your vaccinated chicks, if they got it. But, if they did get it, they would probably all get it together any way.
Weird stuff, and I am not completely comfortable that I have a firm grasp on all the issues, but I think what I have written thus far is accurate.
UGCM