Confirmed cases have plummeted in North Carolina in the past four days, down from a record high of 1,768 on 6/12 to well below trend at 751 today 6/16. Does this mean the actual threat of the disease is also plummeting? It could instead reflect that testing also declined sharply over that interval, from a high of 21,822 on 6/12 to 12,942 today.

I made this same point in the opposite direction almost two weeks ago (see my post here). Taking a look at case trends in May and early June up to that time, it is very interesting that North Carolina experienced five consecutive record high case numbers on five consecutive Saturdays starting May 16. Why Saturdays?

Well, turns out, North Carolina had spikes in completed tests on five the preceding Thursdays and Fridays prior. That late-week pattern of more tests and more cases became even bigger starting with Saturday May 30th spike. The two time series dip and spike almost in lock step.

Here we are on a Tuesday, however, and we’ve had a 40% drop in testing and a 60% drop in cases in just four days. Does this mean the threat of coronavirus is down by the same amount? Almost certainly not. Instead, it means that the factors determining testing are down. Some of those factors do correlate with the seriousness of the disease, because when more people are symptomatic then more people will be getting tested.

However, so much of the variation in test and case numbers fails to measure variation in the actual threat of the disease, but instead picks up lots of variation in human behavior and official decisions. For example, the state’s testing guidelines were initially written when test supplies were pretty tight. As more became available, the guidelines expanded. As weeks went on, more recently the guidelines were expanded again to include testing on marginalized populations disproportionately harmed by the virus, regardless of symptoms.

There is much more to the story that tells the tale of testing. For the moment, it’s important to recognize that the sharp dip in cases in recent days has been caused at least in part by the sharp dip in testing that preceded it.

To their credit, the Governor and NCDHHS place more weight on hospitalizations than they do on cases. However, there is room for improvement because with last weekend’s case spike, Newsweek quoted Governor Cooper as saying, “These [spiking case] numbers show the disease is spreading, and that more people need hospital care.” Meanwhile hospitalizations are almost level between 6/12 and 6/16.

If the pattern holds, expect testing and cases to spike again late this week, followed by alarming headlines, followed by officials having to gently talk it down and focus the public’s attention instead on the counts that matter more, namely hospitalizations and fatalities.