IFNB1 and infection: In order to analyse whether the reduced replication of (A149V) and (L144A, L146A) mutant viruses is the result of an impaired ability of NS1 protein to block host cell interferon production, we infected A549 cells with the 5 mutant viruses and the wild-type virus at different MOI values and measured the production of IFN-β and IFN-λ1 into the supernatants at 20 h post infection (Figure 6A and B).