Protective Efficacy Evaluation of Various Inactivated Vaccines Against the Newly Circulated Highly Pathogenic Avian Influenza Virus H5N1 of Clade 2.3.4.4b in Pekin Ducks

Highly pathogenic avian influenza (HPAI) virus H5N1of clade 2.3.4.4b has emerged as the predominant lineage circulating in poultry flocks worldwide, raising concerns regarding the protective efficacy of currently available commercial vaccines, particularly in domestic ducks, which play an important role in virus maintenance and transmission. Thus, this study evaluated the immunogenicity along with the protective efficacy of four inactivated H5 vaccines against a recently isolated local HPAI-H5N1 (Newvalley-3-H5N1-2024, clade 2.3.4.4b) strain in Pekin ducks in Egypt. A total of 150 seronegative ducks were divided into vaccinated and control groups (10 groups) and vaccinated at 10 days of age. At 31 days of age, the vaccinated and positive control groups were challenged using 106.5 EID50/0.5 mL/duck with the local isolate (Newvalley-3-H5N1-2024) via the oculo-nasal route. The vaccine efficacy was assessed through clinical signs, survival rate, hemagglutination inhibition (HI) antibody titer, tracheal and cloacal viral shedding quantified by real-time RT-PCR, and histopathological examination of trachea, lung, pancreas, and brain tissues. Generally, all ducks vaccinated with the ValleyVac Avian Flu H5 plus and MEFLUVACTM H5 PLUS 8 showed a significantly higher survival rate (100%) at 10 days post-vaccination (DPV) than those in the positive control (66.7% mortality rate). In contrast, ducks exhibited mortality rates ranging from 6.7% in the SERVAC Flu H5N1 group to 13.4% in the Sinder Fluvac group. The ValleyVac Avian Flu H5 plus and MEFLUVAC™ H5 PLUS 8 vaccines induced the highest HI antibody titers at 7, 14, 21, and 28 DPV in both homologous and heterologous AIV antigens, resulting in a significant reduction in viral load among all vaccinated duck groups (p-value < 0.05) comparable to the positive control group. Conversely, the SERVAC Flu H5N1 and Sinder Fluvac vaccines provided partial protection, suboptimal immunogenicity at different time points, and elevated viral shedding. Histopathological findings in ValleyVac Avian Flu H5 plus and MEFLUVAC™ H5 PLUS 8 vaccines exhibited mild tissue alterations following AIV challenge. Marked pathological lesions were observed in the SERVAC Flu H5N1 and Sinder Fluvac vaccinated groups. Among tested vaccines, both ValleyVac Avian Flu H5 plus and MEFLUVACTM H5 PLUS 8 showed the highest level of protective efficacy against the circulating AIV strain compared with other commercial vaccines. This study highlights the need for continuous molecular surveillance, antigenic matching, and regular updating of vaccine seed strains to ensure efficient HPAI control in Egypt.