Figure 7. Fitness of mutant B/Brisbane/60/2008 viruses.

(a) In vitro fitness. MDCK cells were infected with the WT or mutant B/Brisbane/60/2008 viruses at MOI 0.01. Released viral genome in the supernatant was quantitated at day 4 post infection by qPCR of the viral M1 Matrix gene. Genome copy numbers are shown as histograms. The assay was done in triplicate. (b) In vivo fitness. DBA/2 J mice were infected with WT or mutant viruses at 3 × 10⁴, 1 × 10⁴ or 5 × 10³ plaque-forming units (pfu) per mouse. Survival curves (top panels) and percent of average BW as compared with the average pre-infection weight (bottom panels) are shown. Each group contains six mice. (a: mean and s.e.m.; b: mean and s.d.)