Significant differences in nucleotide diversity between intraseasonal periods.

The nucleotide results mirrored the amino acid results. The early epidemic period was more diverse and the peak epidemic period less diverse than the other periods. The null hypothesis that the three periods had equal median diversity was rejected by the non-parametric Kruskal-Wallis test (χ2 = 43.43, p<0.001). These observations were consistent with the results for amino acid sequences, supporting 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.