Failure of a recombinant fowl poxvirus vaccine containing an avian influenza hemagglutinin gene to provide consistent protection against influenza inchickens preimmunized with a fowl pox vaccine

Vaccines against mildly pathogenic avian influenza (AI) have been used in t urkeys within the United States as part of a comprehensive control strategy . Recently, AI vaccines have been used in control programs against highly p athogenic (HP) Al of chickens in Pakistan and Mexico. A recombinant fowl po x-AI hemagglutinin subtype (H) 5 gene insert vaccine has been shown to prot ect specific-pathogen-free chickens from HP H5 AI virus (AIV) challenge and has been licensed by the USDA for emergency use. The ability of the recomb inant fowl pox vaccine to protect chickens preimmunized against fowl pox is unknown. In the current study, broiler breeders (BB) and white leghorn (WL ) pullets vaccinated with a control fowl poxvirus vaccine (FP-C) and/or a r ecombinant fowl poxvirus vaccine containing an H5 hemagglutinin gene insert (FP-HA) were challenged with a E-IP H5N2 AIV isolated from chickens in Mex ico. When used alone, the FP-HA vaccine protected BE and WL chickens from l ethal challenge, but when given as a secondary vaccine after a primary FP-C immunization, protection against a HP AIV challenge was inconsistent. Both vaccines protected against virulent fowl pox challenge. This lack of consi stent protection against HPAI may limit use to chickens without previous fo wl pox vaccinations. In addition, prior exposure to field fowl poxvirus cou ld be expected to limit protection induced by this vaccine.