Immunogenicity and protective efficacy of inactivated and vectored H9N2 vaccines in broilers with maternally derived antibodies

In Morocco, low pathogenic avian influenza H9N2 first appeared in 2016, causing substantial economic damage to the poultry industry. Vaccination is the primary strategy for controlling its spread. This study compared, under experimental conditions, the protection offered by an inactivated (vaccine A) and a vectored (vaccine B) H9N2 vaccine in chickens. 150 one-day-old chicks were split into five groups. Group A and A′ received vaccine A at one and seven days old, respectively. Group B received vaccine B at one day old. Group C was unvaccinated but challenged, and Group D was an unchallenged control. At 28 days of age, all but Group D were challenged with 10 7 EID 50 of A/broiler/Morocco/172-2022 (H9N2) virus via intranasal route. Clinical signs were monitored, macroscopic and microscopic lesions were investigated, seroconversion and virus shedding features were evaluated. Results showed that Group A′ had the mildest respiratory signs. Group B showed higher mean macroscopic lesion scores than Groups A and A′, while Group C exhibited the most severe microscopic alterations. Birds in Groups A and A′ shed significantly less virus than birds in Group B, which had a high percentage of birds shedding virus for nine days or more. The unvaccinated, positive control birds in Group C had the most severe lesions and highest viral shedding. Additionally, Group A′ showed a stronger antibody response than other vaccinated groups. In conclusion, the inactivated vaccine was most effective when administered at seven days of age, resulting in fewer clinical signs, better seroconversion, and faster viral clearance compared to the vectored vaccine or the same inactivated vaccine given at one day.