Vaccine failure takes place when birds is not properly protected after receiving a vaccine and, therefore, contracts the disease anyway. Vaccine efficacy can be evaluated through ELISA tests that quantify antibody production levels in blood serum. Many variables can reduce vaccines’ efficacy and can be related to the environment, the management practices, the bird or the vaccine.

The most important causes are:

• Related to the vaccine: inadequate storage conditions or application methods can cause vaccine failures. Also, choosing a vaccine strain different from the field’s without cross-immunity or a type of vaccine that is not the proper one. Manufacturer and professionals’ indications must always be considered to design an effective vaccination program.
• Maternal immunity: in young chickens, maternal antibodies may interfere with the replication of live vaccines and reduce their efficacy. To prevent it, the breeder vaccination program should be considered when designing broilers vaccination program’.
• Immunosuppression:  it can be caused by many factors that are frequent in intensive production systems. It may appear due to productive stress, since selection for highly production breeds causes an overstraining of the organs and the immune system, making animals weaker; management, environmental and nutritional factors; and immunosuppressive diseases.