High Level of Protection Induced by Two Fowlpox Vector Vaccines Against a Highly Pathogenic Avian Influenza H5N1 Challenge in Specific-pathogen-free Chickens

The objective of the study was to compare efficacy of two fowlpox (FP) vector vaccines (FP-AI) against H5N1 highly pathogenic avian influenza (HPAI): one (vFP89) expressing the native hemagglutinin (HA) gene from H5N8 A/turkey/ Ireland/1378/83 and the other (vFP2211) expressing a modified synthetic HA gene from H5N1 A/chicken/Indonesia/7/2003. Four groups of 20 1-day-old specific-pathogen-free chickens were made: Groups 1 and 2 were immunized with 3 log10 tissue-culture infectious dose 50% (TCID50) of vFP89 and vFP2211, respectively, whereas group 3 was immunized with vFP89, but received a booster immunization at 2 wk of age with an inactivated vaccine containing A/turkey/Wisconsin/68 H5N9 virus (inH5N9); group 4 was left unvaccinated. Ten birds from each group were challenged on day 21 with A/turkey/Turkey/1/2005 clade 2.2 H5N1 HPAI virus. The 10 other chickens from each group were put in contact with their groupmates on day 22. FP-AI induced low hemagglutination inhibition (HI) titers before challenge (GMT < 4 log2) and an HI titer boost was observed 1 wk after the inH5N9 boost. All directly challenged and 9/10 nonvaccinated contact chickens died after challenge (mean death time of 2.3 and 6.1 days, respectively) and most of them shed virus before death via cloacal and buccal routes. All vaccinated birds were clinically protected from HPAI challenge. One (vFP2211), 2 (vFP89+inact.), or 3 (vFP89) out of the 10 directly challenged vaccinated chickens shed virus via the buccal route 2-5 days postinfection. No shedding was detected in the contact-challenged vaccinated birds. Altogether, these data show excellent levels of protection in all three vaccinated groups, and therefore no detectable effect of the origin of the inserted H5 gene on protection under these tested conditions.