If you put the vaccinated chicks in there and your flock has Marek's they can get infected before the vaccine takes effect. The same is true for chicks vaccinated for Coccidiosis, if your flock has Coccidiosis the effects of the vaccine can be negated by the chicks getting sick before the vaccine can take effect.So in summary, you're saying DONT put days old, Merek's vaccinated chicks in a brooder in a coop where the older flock will get exposed.... Yes ?? Obviously asking about this scenario because it is precisely what I DO, do......
I am not a medical professional but I'd expect this to be true for many vaccines, for chickens as well as for humans.