My chicks are double vaccinated. They get the first vaccine at 1 or 2 days and the second at 28 days. My chicks that are 2 weeks old will only be vaccinated once though. It is all part of my vaccine experiment. Of course i won't have a "control group" but I currently have 18 vaccinated chicks from 6 - 20 weeks that are in with Marek's flock and we'll just see how it goes.