The mouse-lethal-dose-50 (MLD50) was not affected upon infection with SGRs containing the viral polymerase subunits PB2 or PB1 (SC35-PB2SC35F, SC35-PB1SC35F) or the NA surface gene (SC35-NASC35F) compared to the parental SC35 (Figure 1). Here, XK is linked to infection.