Feasibility analysis and study on immune effects of combined immunization with mixed inactivated vaccines in chickens

Combination immunization offers several benefits, such as reducing vaccination frequency, minimizing stress responses, and lowering labor costs. This study aimed to develop a combined immunization strategy using mixed vaccines, and to analyze the feasibility of incorporating inactivated vaccines into this regimen. Furthermore, the study validated the practical effects of combination immunization with a mixture of multiple inactivated vaccines. The study is composed of three parts. Initially, the ND-AI H9 bivalent inactivated vaccine and the AI H5/H7 trivalent inactivated vaccine were mixed. The physicochemical properties of the resulting suspension, including particle size, stability, and endotoxin levels, were analyzed to assess the feasibility of the mixed vaccine. Then the mixed vaccine was administered to SPF chickens, and antibody titers were measured to preliminarily evaluate the immunization efficacy of the combined vaccination approach. Finally, the combined immunization strategy was implemented in a large-scale layer farm, with antibody titers being monitored to assess its immunization effectiveness in a production setting. After mixing the ND-AI H9 vaccine with the AI H5/H7 vaccine, the particle size of the suspension fell between the two original vaccines, with the similar distribution pattern as the single vaccines. After seven days of storage, the suspension showed no separation, no microbial contamination, and very low endotoxin levels. When SPF chickens were immunized with the mixed vaccine, the antibody titers against ND and AI had no significant difference compared to that of single vaccine immunization. Following the application of the combined immunization program using the mixed vaccine in a large-scale layer farm, the neutralizing antibody levels for the targeted disease met expectations, with high positivity rates and low coefficients of variation. These results demonstrate that the combined immunization, employing mixed inactivated vaccines for poultry, is feasible, easy-to-implement and effective. It reduces injection frequency and stress, conserves resources, and offers a rapidly efficient method for widespread application. This strategy also holds significant implications for the development of multivalent inactivated vaccines. It should be noted that all vaccines utilized in the present study were inactivated oil-emulsion poultry vaccines, the administration of which was executed in accordance with the farm-specific immunization schedule and under the direct supervision of the veterinarian through a combined immunization protocol.