IFNB1 and infection: Analyzing the accumulation of type I IFN in the supernatants of infected A549 cells, we noticed that the poorly growing avian H5N1 strains induced an up to 15 times stronger IFN-β secretion from human cells than the human H5N1 and seasonal viruses, regardless whether infection was initiated at low (24 and 48 hpi) or high (16 hpi) multiplicity (Figure 1D and E).