Comparison of the virulence of three pandemic H1N1 strains in C57/BL6 mice. Viral strains NC2, NC8 and NC11 were analyzed in C57/BL6 mice (n = 10/group). Viral infection was established with 105 EID50 of NC2, NC11 and 106 EID50 of NC8 and animals were observed up to 14 days. (a) Kaplan Meier’s analysis shows overall difference in survival curves (P 0.0007). Log-rank sum test shows differences between mortality curves: NC2 vs NC8 (P < 0.0001), NC2 vs NC11 (P 0.0195) and NC8 vs NC11 (P 0.0007). (b) Weight loss curve for the group of animals infected with NC2, NC8 and NC11. Significant differences were observed at 3 d.p.i., NC8 vs NC2 (P < 0.0001), NC8 vs NC11 (0.0007).