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The M1-CovS-WT strain is more virulent than the M1-CovS-Y39H mutant strain. Mice were infected with the M1-CovS-WT and M1-CovS-Y39H strains intravenously (n = 10) at 4 × 107 and 5 × 107 CFU, respectively (A), and intranasally (n = 6) at 5 × 108 and 1.2 × 109 CFU, respectively (B). The survival of the animals was scored for a period of 14 days. Dashed lines, M1-CovS-WT; dotted lines, M1-CovS-Y39H. **, P < 0.001 (by log rank and Gehan-Breslow-Wilcoxon tests).
