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Leading and Lagging Indicators

The order of events in a COVID-19 case which does not recover is basically:

  1. Exposed

  2. Tested

  3. Hospitalized

  4. Death

The count of new cases comes from the outcomes of tests. Depending on when the victim chooses to go to the doctor, some time lag results before the case is bad enough to result in hospitalization. Then some more time passes before death or recovery occurs.

There are *many* confounding factors in daily case counts, but in the most general sense, if cases go down, then after some period of time, deaths should go down as well. It is possible to get a rough idea what the average time lags are by watching where each curve (graph of the daily numbers) changes direction, and looking at the number of days between these points. The logic of the progression of the disease is not arguable. Cases first, deaths later. 


But the curves are not making sense right now. Cases reached a high plateau recently in mid-July. By July 24, the averaged daily case numbers began to decline.

The deaths also rose, with the rise beginning later than the rise in cases, because of the natural progression of the disease. 

The cases peaked, and then began declining around July 24. This inflection point gives us a marker, after which we would expect to see deaths decline, following some time lag.

The deaths also peaked and plateaued - but the plateau lasted a *long* time, 3 weeks altogether, from the end of July through mid-August. And daily deaths have declined only gradually in the last few days.

Altogether, the daily deaths have declined only 11.6% in the 26 days since hitting their peak.

The comparison between these two curves should take into account the time lag. We *do* expect deaths to decline later than cases.

If we look at the point of mode change, there is a lag of about 10 days between the cases peaking, and the deaths peaking. 

So to compare the size of the case decline, we need to take the 26 days after its peak, 26 days from July 24. 

But when we look at that period, July 24 through August 18, we see that cases declined 26.9%. Why is the decline in the number of cases so much greater than the decline in deaths??

It should be the other way around. 


Doctors are supposed to be applying improved treatment protocols which save lives. I actually do not doubt that this is the case. The medical providers are doing everything they can, and it makes a difference. This makes the explanation of the discrepancy *harder* to explain.

One problem the USA is facing right now is that testing is *not* progressing as it should. To get control of COVID-19, the nation should be completing 5 million tests per day. Until the end of July, we were far short, but at least heading in that direction. After that, the number of tests given began to fall. 

This could partially answer the question. If cases are in fact falling (they probably are, at least somewhat), and at the same time fewer tests are being completed, then lack of testing would exaggerate the decline in cases.


There's no getting around the deaths. Those are facts on the ground. They stayed high for far too long, and they have declined far less than we would expect by this time. 

There are additional possible explanations. News reports in July talked about victims waiting in line for hours to get tested, and then for days to get the results. The huge effort required to get tested at all, and the relative uselessness of tests which come back many days later, discourage the public from seeking tests at all. 

Hopefully we will see the daily deaths decline more steeply. Maybe they will 'catch up' with pace of declining cases, at least partly. 

Right now we have an anomaly, a stubbornly high pace of daily deaths which suggests that other things are going on, which are not reflected in just the case counts.  

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