All experiments were performed on ST6GalI-expressing MDCK cells [29] which over-express the a -(2,6) sialic acid receptor predominantly found in the human upper respiratory tract. Prior to infection, cells were grown to confluence, achieving an average diameter of *20mm (used herein as a unit of length, d cell ). Plaque assays were prepared using a semi-solid overlay of 1.2% Avicel RC-581 (FMC Biopolymers, Newark, Delaware, USA) as described by Matrosovich et. al. [57] and stained with crystal violet. Six-well plates (Corning Life Sciences, Lowell, MA, USA) were infected with 25+10PFU=well, representing a multiplicity of infection (MOI) of approximately 10 {5 , and stained every 12 h for 96 h. The plates were then photographed using a DSLR camera (Fujifilm S2 with a 60 mm Nikkor macro objective) and the areas of viral plaques were measured using the Threshold and Analyze Particle features of ImageJ, an NIH opensource image analysis software [58, 59] . All plaque radii at one timepoint (three independent experiments of three wells each) were averaged and the standard error of the mean was calculated. The radial growth rate was determined by linear regression to the average radii at time points prior to 72 h. Multiple-cycle viral yield assays for the A/Brisbane/59/2007 WT and H275Y mutant were performed with MOI&10 {5 . Supernatants were harvested every 6 h for the first 36 h of infection and every 12 h subsequently, then titrated by plaque assay as previously described [22] . The geometric average and standard deviation was determined from three replicates at each time point. High MOI single-cycle yield assays were performed as described by Hurt et. al. [60] . Monolayers of ST6GalI-MDCK cells were grown to confluence in 12-well plates and infected with 10 6 PFU well (MOI = 1) in 1 mL of infection medium. Virus was adsorbed for 1 h at 37 0 C in a CO 2 incubator. The supernatant was then removed and cells were quickly washed once with acidic saline (0.9% NaCl in water, pH 2.2) and twice with PBS (pH 7.4). Fresh maintenance medium was added and plates were returned to the incubator. Supernatants were harvested every two hours for 24 h ( Figure 7A , Experiment 1) or every hour for 14 h (Experiment 2), in duplicate. Samples were frozen at {80 0 C until titrated in duplicate by plaque assay [57] .