MNV-1 (A, B, C) and MNV-3 (D, E, F) were serially passaged in the absence (white circles), or presence of 200 (grey triangles) or 400 μM favipiravir (black squares). Passage 1 infections were carried out at an MOI of 0.1 TCID50/cell. Subsequent passages were carried out with 200 μl (1/10 vol) of neat virus recovered from the previous passage. The different graphs show the resulting virus titres (A and D), genome copy equivalents (B and E), and the resulting specific infectivity determined for encapsidated genomes (C and F). Specific infectivity values were calculated as the number of infectious viruses (TCID50 units) found in 109 genome copies from data obtained in A, B, D, and E. To calculate the number of genome copy equivalents, non-encapsidated genomes were removed before RNA extraction by micrococcal nuclease treatment. The arrow in D indicates virus extinction confirmed by the absence of infectious virus and viral RNA (qPCR) after three serial passages in the absence of favipiravir.