Source: US CDC FluView interactive, 2017-2018 flu season. X-axis is week of the year, so this starts in week 40 of 2017.
I picked this particular flu season because it seemed to have just one major peak. And the question is, do all the states peak around the same time? Or are they scattered.
Here’s the state data, graphed. Underlying data are from the same source. I cut this off around week 25 of 2018.
It’s tough to say whether or not the eye is fooled. It does not appear that all the states peak around the same time. In fact, the two-humped US line (first graph above) is really the result of two different sets of states peaking at two different times.
But there does seem to be a fair degree of agreement across states in the timing of the end of flu season. That’s circled in blue above. You can say that either as “by the time it’s the end of flu season, every state has downward trend” or “by the time every state has a downward trend, that’s signalling the end of flu season”.
Either way, I think I’ve answered the question that I posed in my last post. You can, in fact, have a flu season with a high degree of correlation across states, in the winding-down of the flu season.
So the fact that all the states simultaneously show a drop in COVID cases is, in that context, not that unusual.
But it’s not clear that this is typical, either. If I move to the 2018-2019 season, we again have two peaks, due to two different sets of states. And, again, by the time you’re at the end of flu season, pretty much all the states are showing declines, all at the same time. Just starting from different levels.
What I’m trying to get at is that there does seem to be a fair degree of coordination of the end of flu season, across states. And it’s not a tautology. It’s not as if (e.g.) the Southern states get over it in January, and the Northern states continue the season out to March. To the contrary, you seem to find a few weeks when more-or-less all the states are showing declining flu incidence. And when that happens, that seems to flag the true end of the season.
I think my takeaway is that when the U.S. third wave of COVID is finally over, we probably ought to see all the state COVID rates declining, at the same time. Because that commonly happens for flu season.