Embryos (and day old chicks) are extremely sensitive for contamination when injected. If no strict hygiene is applied during the injection and inclusion of bacteria occurs, either in preparing the vaccin, in the machine or in the enviromnent, serious loss of embryos can occur within 1 or 2 days. It is not uncommon to lose more than 50% of the hach when mistakes with hygiene occur. The type of bacteria that is involved in the contamination is not that important, almost any type of bacteria will diminish the hatch.
Crucial for the succes of in-ovo injection is therefore a strict hygiene during the process, both in preparing the vaccin as in maintaing the machine, controlling the injection procedure and controlling the environment.