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 VECTORMUNE® AI 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 VECTORMUNE® AI [1] 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. 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 VECTORMUNE® AI vaccinated group, R was substantially reduced compared to the control group, namely R = 1 (0.5 – 2.1) or R = 1.9 (0.5 – 5.2) 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, highlighting protection against clinical signs and death. - Vaccinated chickens excreted reduced amount of virus when compared to chickens in 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). - Significant T cell proliferation was observed at 7 dpi in the VECTORMUNE® AI vaccinated groups and higher numbers of CD25+ T cells (activated T cells) were found. The response of the vaccinated chickens, at 24 weeks post-vaccination with VECTORMUNE® AI, 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. The large confidence intervals around the R estimates stress the need for careful conclusions on the effect of the vaccine on transmission 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 [17]. The HI antibody levels (HI-titers) of the chickens in the field were measured at days 150-151 (approximately 21 weeks of age) by taking samples of 120 randomly selected chickens [5]. When exploring the distribution of titers in these samples, around 12% of these chickens had titers log2 ≤5. In comparison, 45% of the 20 randomly selected chickens (23 wk of age) for this experiment had titers log2 ≤5. The higher proportion of chickens with titers log2 ≤5 in the experiment compared to the proportion in the field has likely led to increased transmission in the experimental set up. 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.