Significant differences in amino acid diversity between intraseasonal periods.

The early epidemic period was significantly more diverse than expected relative to the peak epidemic and late epidemic periods. The peak epidemic period was significantly less diverse than expected relative to the early and late epidemic periods. The null hypothesis that the three periods had equal median diversity was rejected by a non-parametric Kruskal-Wallis test (χ2 = 48.89, p<0.001). This observation was consistent with the hypothesis that the epidemic peak was associated with the rapid proliferation of a dominant amino acid sequence. An asterisk indicates that the number of high or low diversity events would occur by chance with probability less than 0.01.