Baloxavir and molnupiravir protect against severe influenza A(H5N1) infection in immunosuppressed hamsters

Highly pathogenic avian influenza A(H5N1) viruses continue to expand their geographic and host range, raising concerns about zoonotic transmission and pandemic preparedness. Immunocompromised individuals often develop severe and prolonged disease, yet evidence guiding antiviral therapy remains limited. Here, we characterized the pathogenesis and assessed direct-contact transmission of an influenza A(H5N1) clade 2.3.4.4b genotype D1.1 virus recently isolated from a human case in British Columbia, Canada, in cyclophosphamide-immunosuppressed hamsters, with normal hamsters included for comparison of infection outcomes and viral shedding kinetics. We also evaluated the antiviral efficacy of oseltamivir (20 mg/kg twice daily for 5 days), baloxavir (50 mg/kg, 2 doses), and molnupiravir (50 or 200 mg/kg twice daily) as monotherapies or in combination (oseltamivir with low-dose molnupiravir) in immunosuppressed hamsters. While normal hamsters showed no mortality and achieved viral clearance, infection in immunosuppressed hamsters caused severe disease characterized by prolonged viral replication, neuroinvasion, and 75% mortality, with no direct-contact transmission detected. In the antiviral efficacy experiment, placebo-treated hamsters exhibited 50% mortality. Baloxavir resulted in 100% survival, shortened viral shedding, reduced viral replication and pulmonary inflammatory responses, and prevented neuroinvasion. High-dose molnupiravir also prevented mortality and achieved viral clearance in lung and brain tissues. Combination therapy resulted in 75% survival and similarly achieved viral clearance in both tissues, with reduced morbidity compared with oseltamivir and low-dose molnupiravir monotherapies. Oseltamivir resulted in 75% survival but failed to prevent severe clinical signs or reduce tissue viral titers, whereas low-dose molnupiravir showed outcomes comparable to placebo. These findings demonstrate the high pathogenicity of a human A(H5N1) isolate in immunosuppressed hamsters, support baloxavir as an effective therapeutic option, and identify molnupiravir-based regimens as promising strategies for severe influenza A(H5N1) infection.