NA-H275Y + PA-I38T infected ferrets also shed virus with sequences consistent with the inoculum genotype, at approximately 100% PA-I38T + NA-H275Y, with the exception of Group I RF1 ferrets whose nasal washes had a mixed proportion of WT and double mutant virus genotypes, which resulted in the infection and shedding of pure (approximately 100%) WT virus to the next recipient ferret (Group I RF2) (Supplementary Fig 7b). Here, XK is linked to infection.