The two experimental bird flu vaccines currently in research in a field study, offer longer term protection to layer hens, but don't prevent mortality after infection completely. "At the age of 24 weeks, the vaccinated layers excreted less virus and were protected against clinical signs, but we did observe some mortality in our transmission studies", says Kim Bouwman of Wageningen Bioveterinary Research (WBVR, part of Wageningen University & Research).
The field study with laying hens was commissioned by the Dutch ministry of Agriculture, Fishery, Food Security and Nature (LVVN) as a follow-up to an evaluation of bird flu vaccines in a laboratory setting. At the start in September 2023, 1800 laying hens were vaccinated and housed on two commercial poultry farms. The field study at these two farms will run until the third quarter of 2025. During this field study, a number of the vaccinated chickens will be brought to WBVR for a transmission study on four occasions. The first of these four studies was conducted 8 weeks after vaccination. It was demonstrated that the layers were fully protected against bird flu at that age. The second study was at 24 weeks of age, the period in which the egg production is stepping up to its peak.
Transmission study
For the transmission study a randomly selected group of chickens from the two commercial farms were brought to WBVR. The layers were vaccinated with one of the HVT-vector vaccines: VECTORMUNE® AI or VAXXITEK HVT+IBD+H5. In addition, a third group was included that received the VAXXITEK HVT+IBD+H5 vaccine at day of hatch, and a booster vaccination at 12-weeks of age. The animals were challenged with highly pathogenic Avian influenza, HPAI H5N1 virus (clade 2.3.4.4b). Transmission from inoculated to contact chickens was determined, as well as survival of the chickens. As a result, the reproduction number (R) was estimated. Furthermore, virus shedding and humoral and cellular immune responses were determined.
Protection
In all vaccinated groups, no clinical signs were observed and a significant reduction in mortality compared to the non-vaccinated control groups. "In our non-vaccinated control groups mortality was 100 percent. In the groups that received only one vaccination, we found a mortality rate of 10 percent. No mortality was observed in the group that received a booster", says Kim Bouwman, at WBVR in charge of the bird flu transmission studies.
Virus transmission
Even though the R-value of vaccinated chickens was lower than the R-values estimated for the control animals, the confidence intervals around the estimated R-values are large. "These findings urge us to be very careful when drawing our conclusions based on this transmission study alone", stresses Bouwman. "In the field, we found that within a vaccinated flock individual animals can greatly differ in the expression of the level of immunity. This influences the transmission and therefore, we can't draw conclusions on protection against sustained transmission without the additional data from later stages of the field study." All vaccinated groups also showed strongly reduced amounts of excreted virus when compared to the control groups.
At the end of the longitudinal study, data from the additional transmission experiments and 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, according to the WBVR researcher. "We need to take these additional data into account before drawing final conclusions." The field study will continue until the third quarter of 2025.
