Preclinical evaluation of a live avian metapneumovirus subtype B vaccine strain with cross-protective efficacy in chickens

Avian metapneumovirus (aMPV) infection is an important respiratory disease that causes significant economic loss in the poultry industry. Here, we aimed to develop an effective live aMPV subtype B vaccine strain suitable for mass vaccination of poultry farms to achieve herd immunity. The wild-type virus was attenuated by serial passaging in Vero-K cells, and the genetic stability of the attenuated virus was evaluated through genomic sequencing analysis. The attenuated virus was formulated into test pilot live vaccines. Preclinical tests of the test vaccine, including those measuring indices of safety, minimum immunogenicity, virulence reversion, immunogenicity and cross-protective efficacy, were conducted in SPF chickens. The master seed virus (MSV) of the SNU21004(a) vaccine strain was established by serial passaging in Vero-K cells. Some point mutations occurred throughout the viral genome, but no gene deletions or insertions were observed. The clinical scores, histopathological scores, and viral loads of SPF chickens inoculated with the MSV were significantly lower than those inoculated with the wild-type virus (P < 0.05). There was no increase in viral virulence after five backpassages in chickens. The minimal immunogenicity dose for protection against aMPV is 10^2.1 TCID₅₀/dose. The test vaccine induced a strong humoral immune response and efficiently protected chickens, even in vaccinated birds with low antibody titers, regardless of the route of administration (drinking water or spraying) or challenge virus subtype (subtypes A and B). The SNU21004(a) vaccine strain was found to be safe and highly immunogenic even at small doses in chickens and showed cross-protection against other aMPV infections.