chicks are very vulnerable to several conditions that might cause their deaths. success in chicks depends on how best you manage your brooding conditions and the brooder itself. kindly note that vaccinations only controls viral diseases. bacterial diseases and other disease causing microoganisms outside vaccination schedules can be managed by proper management protocol which includes and not limited to biosecurity. kindly check on the feed quality and general hygine of your brooder. otherwise you can describe the signs o the dead chick and or invite nearby qualified veterinerian for on farm assesment.