Infection of the human lung epithelial cell line A549 with RSV-HD resulted in the expression of IFNL1 (also known as IL-29), IFIT1 (also known as ISG56) and IFNB1; no expression of these genes was detected upon infection with RSV-LD, despite higher levels of viral replication (RSV G expression) (Fig 4A). Here, IFNL1 is linked to infection.