IFNA1 and infection: Nuclear replication, efficient encapsidation of cRNA and vRNA, and IFN antagonism by viral proteins are very effective at preventing activation of the IFN system during infection: studies examining IFN induction in individual cells infected with a range of strains of influenza A virus found that the IFN-β promoter had only been activated in a small proportion of infected cells, both in vitro and in vivo (Fig. 1) (Chen et al., 2010, Kallfass et al., 2013).