Figure 4. The proportion of influenza B in the pre-pandemic period negatively correlates with that of the post-pandemic era among nine census regions of the United States.

The post-pandemic era is defined from Jan 2010 to August 2015. We vary the start date of the pre-pandemic from 1997 to 2007, and fix the end of the pre-pandemic era to be 2008. It is clear that there is a window period such that the proportion of influenza B shows a significantly negative correlation between pre- and post-pandemic periods. (a) Correlation and p-value as a function of start year of the pre-pandemic period. The x-axis shows the start of the pre-pandemic period. (b) The correlation between 2003–2008 versus 2010–2015.