EF treatment does not select for resistant IV variants. MDCK cells were infected with KAN-1 (MOI = 0.001) and incubated 24 hours either with media without EF (black bars), or containing EF (50 μg/ml hatched bars) or Tamiflu® (2 μM, grey bars). Supernatant was titrated by FFU assay and used for a second round of infection of fresh MDCK cells. Three passages (1st, 2nd, 3rd round) were performed and after each the virus titer (FFU/ml) was determined by FFU assay. FFU titres of EF- and Tamiflu®-treated samples were calculated as percentage of controls set at 100%. Shown is the mean of duplicate experiments titrated in duplicates.