SPF eggs must be used for vaccine production. At 5-6 days of incubation, the microorganism is inoculated into the yolk sac of the embryonated eggs, which are harvested after death of the embryo at 12-15 days. Egg-related allergy is common, particularly in children with asthma or general allergies, and may be as high as 40% in children with moderate to severe atopic dermatitis. The risk of egg-related allergy after vaccination depends on the presence of egg protein in the final product. For example, influenza vaccine is manufactured using the extra-embryonic fluids of chick embryos and contains measurable quantities of egg proteins.