The prevention of H5N1 avian influenza: inactivated low pathogenicity avian influenza virus-based vaccine against high pathogenicity avian influenza virus strain

High pathogenicity avian influenza remains a great challenge for poultry farming industry worldwide. High pathogenicity avian influenza viruses cause devastating epizooties leading to significant economic losses. Moreover, high viral genome reassortment probability could result in abrupt virus pathogenicity expansion and the virus will become dangerous for humans. Considering this, development of specific vaccines against avian influenza is of great importance. The research was aimed at the evaluation of protective effect of “AviFluVac” inactivated vaccine against avian influenza based on H5N1 low pathogenicity AIV Yamal strain antigen. The vaccine was tested for its protective effect by challenge using H5N1 highly pathogenicity AI virus A/gull/Kirov/998-1/2023 strain isolated in Russia in 2023. “AviFluVac” vaccine used in parallel with the pilot vaccine based on H5N1 HPAI virus antigen as a reference vaccine. Both vaccines were tested at different concentrations (D) of relevant antigens. Constant vaccine inoculation volume contained undiluted antigen (D = 1); 1/25; 1/50; 1/100. Vaccine containing each antigen concentration was inoculated to a separate group of chickens (n). Mean logarithmic antibody titres (T, log2) to avian influenza virus were determined in the groups 28 days after vaccination and then chickens were challenged with high pathogenicity avian influenza virus. Proportions of clinically diseased and dead chickens (c) were registered daily in each group. Group clinical scores (C = Ʃс/n) and protective index values (PI = 1 - C) were determined 10 days after challenge. PI values were converted into linear equivalents: f = log(PI/(1 - PI)). Regression models, 'f = k(lgD) + f0, were constructed for tested and reference vaccines and used for calculation of relative antigen concentration required for protection of 50 % of vaccinated chickens (PD50). It was shown that “AviFluVac” inoculation volume contained (1.982±0.088) lgPD50, or ≈ 96 PD50 and reference vaccine inoculation volume contained (1.581±0.122) lgPD50, or ≈ 38 PD50. The calculated values were significantly different (p≤0.01). The study of the relationship between f and T values revealed that the expected antibody titers corresponding to the 95 % protection of vaccinated chickens, were statistically equal (log2) for the AviFluVac vaccine and reference vaccine – 5.85 and 6.09 respectively.