Progress report: transmission study testing HVT-based H5 vaccine against highly pathogenic avian influenza (HPAI) H5N1 virus (clade 2.3.4.4b) : First report, 8-weeks post vaccination with VAXXITEK HVT+IBD+H5

This progress report describes the first transmission study in a series of four studies. This long-term study aims to determine whether vaccination under field circumstances can protect layer flocks long term against HPAI H5N1 virus (clade 2.3.4.4b) and reducing within flock transmission (R<1). This transmission study was performed at 8-weeks post vaccination with layer hens which were housed under field circumstances. In addition to the main output, which is assessment of the level of transmission, other variables such as chicken survival, virus shedding, humoral and cellular immune responses were studied to assess the effectiveness of vaccination with VAXXITEK HVT+IBD+H5 vaccine in poultry against HPAI H5N1 virus infection. The estimated R for the control group was 1.3 (95% CI 0.58–2.87) and transmission in the VAXXITEK HVT+IBD+H5 could not be quantified as none of the inoculated chickens became infected. For the analysis, a chicken is considered infected when the following parameters applied: • Virus shedding: when virus was detected for 2 days or longer (≥2 days) with a minimum equivalent titer of ≥Log 101.7 eqEID50/ml by PCR in swabs collected from either choana or cloaca. and • If the chicken survived the challenge, additional parameters were: o a positive NP-ELISA result (after 21 days) and/or o showed an increase of ≥3 log2 in the heterologous HI-titer. This definition is consistent with the definition used in our previous study [1]. In the control groups 9/10 inoculated chickens and 7/10 contact chickens met these criteria and were considered infected. The non-infected inoculated chicken in control group A, was positive for virus shedding, but had no positive serological response. After vaccination with the VAXXITEK HVT+IBD+H5 vaccine, none of the chickens became infected (although 2/10 inoculated chickens were positive for virus shedding). No positive serological results were obtained. In addition, prior to inoculation, the vaccinated chickens had a significantly higher absolute numbers of T cell (subsets) in the blood. The absolute number of activated T cells in the blood peaked at 7 days post inoculation.