Progress report: transmission study testing HVT-H5 vaccine against highly pathogenic avian influenza (HPAI) H5N1 virus (clade 2.3.4.4b) : second report, 24-weeks post vaccination VAXXITEK HVT+IBD+H5 and VAXXITEK HVT+IBD+H5 + Volvac® B.E.S.T. AI+ND vaccine

Samenvatting

This report describes the results obtained in the second transmission study in a series of four. These transmission experiments are part of a longitudinal study which aims to determine whether vaccination of laying hen flocks under field conditions can provide long-term protection against HPAI H5N1 virus (clade 2.3.4.4b), especially against virus transmission (within-flock reproduction number R<1) measured under experimental conditions. In the first transmission study at 8 weeks post-vaccination with HVT-vector vaccine VAXXITEK HVT+IBD+H5 [2] the pullets were protected against clinical signs and transmission following challenge. For this second study a (random) selection of chickens, around time of peak egg production, were transported to high containment experimental facilities for the challenge with HPAI H5N1 virus (clade 2.3.4.4b). Up to that moment, the chickens were vaccinated and housed under field conditions. The layers were challenged with HPAI H5N1 virus (clade 2.3.4.4b) 24 weeks post-vaccination with the vector vaccine VAXXITEK HVT+IBD+H5 (and 12 weeks post-booster vaccination with Volvac® B.E.S.T. AI+ND). Transmission from inoculated (challenged by inoculation) to contact chickens was determined, as well as survival of the chickens, virus shedding and humoral and cellular immune responses for 21 days post-inoculation (dpi). The key findings in this study were: - In the non-AI vaccinated control groups all chickens (20/20 inoculated and contact) became infected after challenge, and the estimated reproduction number was significantly >1, namely R (95% Confidence Interval) = 15.4 (6.0 – 33.0). In the VAXXITEK HVT+IBD+H5 vaccinated group, R was substantially reduced, namely R = 7.9 (3.2 – 16.3) or R = 2.8 (1.1 – 5.7) depending on the estimation method. Only in the VAXXITEK HVT+IBD+H5 + Volvac® B.E.S.T. AI+ND group R-values <1 were estimated, namely R = 0.6 (0.2 – 2.2) or R = 0.3 (0.1 – 1.0) depending on the estimation method. - Whereas 100% mortality by 5 dpi occurred in inoculated and contact-infected chickens in the non-vaccinated control groups, mortality was significantly reduced to 10% in the vaccinated groups. When chickens received a booster vaccination at 12 weeks of age, no mortality was observed, highlighting protection against clinical signs and death. - Vaccinated chickens excreted reduced amount of virus when compared to chickens in the control groups. - The VAXXITEK HVT+IBD+H5 + Volvac® B.E.S.T. AI+ND group, excreted reduced amount of virus when compared to chickens in the control groups particularly through the cloaca compared to the control groups. - Serological responses post-inoculation indicated that the majority of vaccinated chickens developed antibodies in response to challenge (based on results of NP-ELISA and HI titers at 21 dpi). - The total number of T cells demonstrated a tendency to increase after challenge; however, no significant differences were observed in number of T cells when compared to 0 dpi. At 7 dpi, the number of CD25+ T cells (activated T cells) was significantly higher in chickens vaccinated with VAXXITEK HVT+IBD+H5 + Volvac® B.E.S.T. AI+ND when compared to the VAXXITEK HVT+IBD+H5 vaccinated chickens. Additionally, immune cells from VAXXITEK HVT+IBD+H5 + Volvac® B.E.S.T. AI+ND vaccinated chickens produced IFNγ in response to the challenge virus, suggesting a strong anti-viral immune response. The response of the vaccinated chickens, at 24 weeks post-vaccination with VAXXITEK HVT+IBD+H5 and 12 weeks after booster vaccination with VAXXITEK HVT+IBD+H5 + Volvac® B.E.S.T. AI+ND, demonstrated enhanced survival, and stimulated humoral and cellular immune responses against challenge with HPAI H5N1 virus (clade 2.3.4.4b) compared to non-AI vaccinated controls. Only the group that received a booster vaccination at 12 weeks of age, had a R-value < 1. However, the large confidence intervals around the R estimates, stress the need for careful conclusions based on the data of this study alone. In vaccinated flocks, transmission (R) is influenced by the proportion of chickens expressing low and high levels of immunity. In experimental setting, a small proportion of chickens with low neutralizing antibody titers can significantly increase transmission (R>1) due to higher infectivity. Since the distribution of antibody titers in the field and how these change over time are not available yet, more detailed insights into vaccine effectiveness require integration of all field data and transmission studies. Therefore, it is not yet possible to draw conclusions on protection against sustained transmission without the additional data from later stages of the field- and third and fourth transmission studies. At the end of the longitudinal study, the additional transmission experiment data and HI titer distribution in the field flock over time will enable a more accurate quantification of transmission and predictions of the duration of protection over time. In the final report, the combined data will be presented to support conclusions on effectiveness of a large-scale single dose application of this vaccine to stop sustained transmission, and hence on its potential as preventive measure to control HPAI for the whole of the production cycle.