I was reading that the first round at hatch, the second round at 6 weeks. I was also reading another study that suggests even adult birds who've not been vaccinated can be vaccinated, and that there is some indication that vaccinating birds that are starting to show symptoms can stop the symptoms. But THAT is not yet proven, just theory for the moment. Needless to say, for the cost of the vaccine, well for the one, is only 20 bucks. Then there is the other that is 60. Now if only it was just like... use what I need, then put the rest in storage... Till I need it again for another batch.