How Over-testing can Fool Us

Creating alarm over artifacts

I was inspired to write this post after someone sent me a public health report from Michigan¹ from October 2021 that had graphed cases by age during the back to school period. The report highlights what appeared to be an alarming trend among school age children during the period when schools opened. The context of this rise was during the Delta variant wave, when cases in all ages were certainly rising. There was a dramatic rise in cases in kids, in comparison to the rise in cases in other age groups.

Crazy, right? Clearly this is evidence that because schools are back in session, that there MUST be high degree of transmission in schools, or that somehow the biological mechanisms of SARS-COV2 have suddenly changed to specifically target kids, right?

Maybe. Perhaps there’s more than meets the eye.

When I saw this I was immediately reminded of the Fall 2021, during our Delta wave (which had started a full month before schools opened), when our local Covid czar and media personalities started raising the alarm about kids making up a higher proportion of cases than ever before. Here’s a few of those examples:

Alex Jahangir @alexjahangir

There are now 5060 Nashvillians currently infected with COVID. Most alarming is the trend in 0-17 year olds. In the past 30 days this group made 21% of all cases. To put this in perspective up until this past month this age group made up 11% of our overall cases. Pls stay safe.

Kimberlee Kruesi @kkruesi

“Right now, 36% of all of our cases in the state are among children when it’s historically been in the 10 to 15% range.” Read more:

apnews.comHealth chief: Children now 36% of Tennessee’s virus casesNASHVILLE, Tenn. (AP) — Children now make up 36% of Tennessee’s reported COVID-19 cases, marking yet another sobering milestone in the state’s battle against the highly contagious delta variant, Health Commissioner Lisa Piercey said Wednesday.

What both of these tweets keyed in on was not that cases were increasing, but that children cases were increasing faster, and just in time for school! A Facebook post from a local parent who crunched the numbers caused concern among parents where I live. I suspect this post is just one example of many like it in places that had larger Delta waves timed perfectly

Looking at the comments you can tell the assumption, or dare I say conclusion, is that school openings must have caused this.

Let’s look a little deeper.

One of the key issues that has plagued (pun intended) our epidemiological analysis is disproportionate testing. If you test one population or subset of the population (in this case, children returning to school), at twice the rate of the rest of the population during times of epidemic spread, you will find more cases at twice the rate of the rest of the population.

Let’s look at the raw data behind the above tweets and Facebook post and see if we can discover what’s really going on.