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The vaccine stops the tumors that cause the damage from forming. They can catch and spread Marek's but Marek's does not harm them.

Where did you get those older chicks? Have they been exposed to other chickens? For the vaccine to be effective the chicks need to be kept from other chickens that have Marek's for a couple of weeks. I don't know how likely it is that your older chicks have been exposed. If they haven't...