Efficacy of a fowlpox-vectored avian influenza H5 vaccine against Asian H5N1 highly pathogenic avian influenza virus challenge.

A recombinant fowlpox-avian influenza (AI) H5 vaccine (rFP-AIV-HS) expressing the hemagglutinin of the A/turkey/Ireland/1378/83 H5N8 AI isolate has been used in Central America since 1998 to control H5N2 low pathogenicity AI. Previously, this vaccine was shown to induce full protection against a panel of H5 highly pathogenic (HP) AI isolates, including HPAI H5N1. Here, we evaluate the efficacy of rFP-AIV-H5 against escalating doses of HPAI H5N1 A/chicken/ SouthKorea/ES/03 isolate and against the HPAI H5N1 A/chicken/Vietnam/0008/2004 isolate. In both studies, 1-day-old specific pathogen-free (SPF) chickens were vaccinated by subcutaneous route with rFP-AIV-H5 and challenged 3 wk later by the oronasal route. In the first study, full protection was observed up to a challenge dose of 6.5 log10 embryo infectious dose (EID50), and the 50% chicken infectious dose was estimated to be 3.1 and 8.5 log10 EID50 in the control and the rFP-AIV-H5-vaccinated group, respectively. A 2-4 log10 and > 4 log10 reduction of oral and cloacal shedding was observed in rFP-AIV-H5 vaccinated birds, respectively. The rFP-AIV-H5 vaccine induced hemagglutination inhibition antibodies (5.2 log2) detectable with homologous H5N8 antigen. In the second study, rFP-AIV-HS-vaccinated chicks were fully protected against morbidity and mortality after challenge with the 2004 Vietnam isolate, whereas unvaccinated chickens died within 2 days of challenge. Shedding in cloacal swabs was detected in all unvaccinated controls but in none of the rFP-AIV-H5-vaccinated chickens. Together, these results confirm the excellent level of protection induced by rFP-AIV-H5 in SPF chickens against two recent Asian HPAI H5N1 isolates.