I have vaccinated my bantams myself at 1 day of age. It does not negatively affect them. Ideal vaccinates bantams so it can't be that harmful. I got my first chickens last year and there had never been any chickens on this land. I was very careful where I got my chickens from...and still, I am fighting Marek's. I have no idea where it came from and so far I have lost almost 30 chickens...I only wish that I could go back and purchase only vaccinated chicks. Read some of the threads on here about Marek's and make your own decision. Marek's is everywhere...I read somewhere that if your chicken breaths, it has been infected. No one is safe
I just had to put down two more of my babies yesterday...another of my favorites....it is a horrible disease that just doesn't quit!