We had our chicks vaccinated before they were shipped to us, I assume for Marek's. We were told to feed NON-medicated feed, so that's what we did. (They all got pasty butt.) I still do not understand the connection between the vaccination and the medicated feed, but we did not lose any chicks to illness and they are now about 14 weeks old and doing great. I can't wait for my first egglets!