A live attenuated NS1-deficient vaccine candidate for cattle-origin influenza A (H5N1) clade 2.3.4.4.b viruses.

Avian Influenza viruses (AIVs) present a public health risk, especially with seasonal vaccines offering limited protection. AIV H5N1 clade 2.3.4.4b has caused a multi-state outbreaks in the United States (US) poultry and cattle since March 2024, raising pandemic concerns. We developed a nonstructural protein 1 (NS1)-deficient mutant of a low pathogenic version of the cattle-origin human influenza A/Texas/37/2024 H5N1, namely LPhTXdNS1, and assessed its safety, immunogenicity, and protection efficacy. LPhTXdNS1 is attenuated in vitro, showing reduced replication efficiency in Vero cells and inability to control IFNβ promoter activation. The LPhTXdNS1-immunized C57BL/6 J mice exhibit significantly reduced viral replication and pathogenicity compared to those infected with the low pathogenic version expressing NS1, namely LPhTX. Notably, a single intranasal dose of LPhTXdNS1 elicited protective immune responses, providing robust protection against lethal wild-type H5N1 challenge. These results demonstrate that LPhTXdNS1 is safe and able to induce protective immune responses against H5N1.