To evaluate the impact of the NA H275Y substitution on viral growth in vitro, humanised Madin-Darby canine kidney (MDCK) cells (i.e. hCK cells) [9] were infected with either a representative NA H275Y mutant virus (A/Yokohama/36/2024) or a recently circulating wild-type virus (A/Kanagawa/IC2401/2024) at a multiplicity of infection of 0.001 focus-forming units per cell. Here, XK is linked to infection.