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The epitope evolution and antigenic drift. (A) The distributions of variant ratios of WER and Smith vaccine strains from 1982-1983 to 2009 seasons. 10 seasons with emerging variants and followed by the update of WER strain in the next season are labelled (red arrow) (B) The average hamming distances (HD) of 5 epitopes from 1982-1983 to 2009 seasons.
