This definition appears rarely
These vaccines, however, may not prevent infection in some individual birds, and if infected could shed virulent virus. Nevertheless, the amount virus shed is considerable less than that of non-vaccinated and infected birds. It is imperative that the circulating antigenic avian influenza virus is known and the vaccine represent this antigenic strain, since there is no cross-protection among the 15 known HA subtypes. A recombinant fowl pox virus vaccine.