Comparison of viral titers from viral supernatants collected from Calu-3 cells infected with A(H1N1)pdm09 for 48 h in the presence of increasing concentrations of OS (A), IFN λ1 (B), and OS plus IFN λ1 (C), this over 12 consecutive passages in Calu-3 cells. Each population is shown together with the untreated control, serially passaged in the absence of selective drug pressure. The titer (performed in MDCK cells) is expressed in plaque-forming units per milliliter (log PFU/mL). The drug concentration at each passage is also shown (D). The results represent the mean and standard deviation from two independent titrations. *, p ≤ 0.05; **, p ≤ 0.01; ***, p ≤ 0.001; ****, p ≤ 0.0001.