Post #920: Now that’s a surge.

Posted on December 16, 2020

Source:  Analysis of data from Centers for Disease Control FluView Interactive.

The graph above shows the development of a typical U.S. flu season.  It averages all U.S. flu seasons from 1997 to 2020.

The vertical axis is the percent of outpatient health care visits that are for “influenza-like illness” (so 1.5 = 1.5% of all visits).  The horizontal axis is the week of the year.  (The whole year would run from 1 to 52, but I’ve only shown July to December here.)  The data are from healthcare providers participating in the CDC U.S. Outpatient Influenza-like Illness Surveillance Network (ILINet).

I’ve drawn in a black trend line.   Assuming your eyes work like mine, you’ll note sharp break from trend starting somewhere around week 49.  You don’t have to guess at it.  It’s not somehow drowned out by the existing trend.  It’s easily visible to the eye.  That is, in a typical year, there a big surge in flu cases starting at that time.

Week 49 is two weeks after Thanksgiving.  More-or-less. 

This is what a plausible post-Thanksgiving surge in cases looks like.  Here, it’s flu cases.  And this is what I keep looking for — and failing to find — for COVID-19.

It’s totally unsurprising that our public health establishment expected to see a surge in COVID-19 following Thanksgiving.  Not only did that happen with the 1918 Spanish Flu, such a surge also occurs, on average, with garden-variety seasonal flu.

We still need to keep asking why we didn’t see that COVID-19 surge.  What did we do right?  Because it would be really good to know that for the upcoming holidays.

It’s not clear yet whether we’ll see any post-Thanksgiving surge in flu cases this year.  The data haven’t been published.  But it’s entirely possible we won’t even have a flu season to speak of, this year.  Here’s a picture of flu incidence in week 49 of the past four years.  Green is low, red is high.  See if you can guess which map is for 2020.