Bovine-derived influenza A virus (H5N1) shows efficient replication in well-differentiated human nasal epithelial cells without requiring genetic adaptation

Highly pathogenic avian influenza H5N1 viruses of clade 2.3.4.4b have caused widespread avian mortality and sporadic mammalian infections, raising concerns about their potential for efficient replication in the human population. Efficient replication in the human upper respiratory tract is considered a key barrier to transmission. Here, we demonstrate that an H5N1 virus isolated from bovine milk in Texas in 2024 (H5N1Tex/24) replicates as efficiently as the 2009 pandemic H1N1 virus (H1N1HH4/09) in well-differentiated human nasal epithelial cells. These cells express both avian- and human-type influenza receptors, indicating receptor adaptation is unnecessary for entry. H5N1Tex/24 replicates effectively at 33 {degrees}C, reflecting nasal cavity temperature, whereas earlier avian H5N1 strains require 37 {degrees}C, suggesting that H5N1Tex/24 has acquired another key adaptive feature to the human upper respiratory tract. H5N1Tex/24 remains sensitive to interferon-{lambda} (IFN-{lambda}) despite inducing low cytokine levels. Notably, no known mammalian-adaptive mutations such as PB2-E627K were detected. These findings suggest that H5N1Tex/24 possesses intrinsic traits enabling efficient replication in the human upper airways, a critical step toward potential airborne transmission, underscoring the need for vigilant surveillance.