Avian influenza virus (AIV) H۹N۲ is endemic in
Iran and its large-scale circulation in the poultry industry of the country is devastating. This virus was first reported in the industrial poultry populations of
Iran in July ۱۹۹۸. Some of the published studies showed that inactivated avian influenza (AI) vaccines are capable of inducing an immune response and providing protection against morbidity and mortality in different countries (Vasfi et al., ۲۰۰۲; Tavakkoli et al., ۲۰۱۱). Low pathogenicity avian influenza subtype H۹N۲ virus has been reported to have a zoonotic potential and widespread distribution in Iran. Therefore, water-in-oil emulsion vaccines are employed to control the disease in chickens (Nili and Asasi, ۲۰۰۳). This cohort study was conducted during July ۲۰۱۶-November ۲۰۱۷ in broiler chicken farms of Qazvin province,
Iran to investigate the serological change trends in broiler chickens in this region. Level of immunity against the H۹N۲ virus was evaluated by hemagglutination inhibition assay. Fifteen farms out of thirty enrolled units used AI H۹N۲ killed vaccines. The minimum of mean antibody titers (MATs) was ۴.۵۴-۲.۴۲ and the maximum of MATs was ۴.۵۴+۲.۴۲ on day ۳. In addition, the minimum and maximum MATs on day ۵۰ were ۰.۴-۰.۶۴ and ۰.۴+۰.۰۶۴, respectively. The transfer rate of H۹N۲ AIV antibodies from the serum of breeders to the serum of chickens was calculated as ۶۰.۳۵% in our study. A significant difference was revealed between the maternal mean antibody titers (MMATs) and the MATs on day ۳ (p <۰.۰۰۱). In addition, the difference between the MATs on day ۳ and the MATs on day ۱۰ was found to be significant (p <۰.۰۱). Moreover, MATs were significantly different between the vaccinated and unvaccinated herds on day ۴۰ (p <۰.۰۵), while no significant difference was observed on days ۳, ۱۰, ۲۰, and ۳۰ (P>۰.۰۵). According to the results of this study, antibody titers in the vaccinated farms did not reach the protective level until the end of the rearing period. Most of the unvaccinated herds experienced a spurt in antibody titers due to exposure to the virus. Consequently, biosecurity measures must be implemented more seriously and strictly in broiler farms.