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Antigenic drift in H1N1 strains is seen in mouse models. Mice were immunized with A/Cal/7/2009 (black squares) and challenged with A/England/195/2009 (A), A/Eng/672/2010 (B), A/England/336/2015 (C), responses were compared to naïve animals (white circles). Antibody response to Cal/09 antigen prior to challenge (D). Points mean of n = 4 mice ± SEM (A–C), or individual animals (D). *p < 0.05, **p < 0.01.
