H9N2 avian influenza viruses (AIVs) are widely distributed, causing continuous outbreaks in poultry and sporadic infections in humans. This study assessed the humoral immune response induced by two inactivated vaccines largely used in poultry. Serum samples were collected from 70 specific pathogen-free chicks at days 21 and 28 post-vaccination and tested against recent avian-derived H9N2 strains from G1 and B (Y280) lineages, isolated in China, Egypt, Benin, Togo, Algeria, and Morocco. The Hemagglutination Inhibition (HI) test was used to measure antibody responses. In Group A (vaccinated with vaccine 1), HI titers showed a significant increase between day 21 and 28. In contrast, group B (vaccinated with vaccine 2) exhibited consistently higher HI titers at both time points but showed no statistically significant increases by day 28 for any strain (all P-values > 0.05). This suggested that vaccine 2 elicited a stronger initial immune response, but with limited further increase over time. Notably, both vaccines induced significantly lower, but still relevant, geometric mean HI titers against the China strain (A/Suzhou/GIRD01/2019) followed by the Algerian strain (A/chicken/Algeria/219/2017) compared to other isolates at both time points. The negative control group (NC) remained seronegative throughout the study. These findings highlight potential antigenic variations among different H9N2 strains and emphasize the need for careful vaccine selection to ensure optimal protection across different geographical regions.
Arbani et al. (Fri,) studied this question.