Coronavirus: Speculation

Does the severity of COVID-19 in the average infected person decrease as the epidemic begins to wane? Daniel Defoe's Journal of the plague year passed along this observation of the resolution of the Great Plague of London in 1665:

"...and you will see many more people recover than used to do; for though a vast multitude are now everywhere infected, and as many every day fall sick, yet there will not so many die as there did, for the malignity of the distemper is abated'"

Now allowing for the fact that Defoe's work was a fictionalized account, and that the disease in question was a bacterial plague, one may nonetheless wonder if disease severity declines as an epidemic or pandemic runs its course. 

Quantifying disease severity is tricky, since there are no reliable, or universally applicable criteria upon which to make such measurements. Patient deaths is an imperfect measure since people may be infected at the peak of the contagion but die near the end. One may assume that the more severely ill patients are more likely to be hospitalized, and this may suggest one surrogate by which we can examine the issue. Here is a chart of the ratio of daily hospitalizations to daily diagnoses in Colorado during the month of April:

It is easy see that the chart tends downward, and this is consistent with a hypothesis that disease severity declines as the epidemic progresses, or begins to resolve. Obviously however, many things could account for the observation. It may be that:

1. more people who are less affected are being tested;
2. the criteria for hospital admission has changed in favor of admitting those who are less ill;
3. people who had been diagnosed on one day might not be admitted to the hospital until several days later;
4. there are anomalies in data collection or reporting;
5. testing is more widespread, encompassing more than those who were initially very sick and who met the original criteria for testing;
6. Other things...

It may also be the case that the effect contemplated above is real. The change in magnitude represents an 80% decrease from April 1 through the present. That is a large change to be accounted for by one or two factors. If the effect is real, it might suggest:

1. that COVID-19 is more widespread and less virulent;
2. that SARS cov-2 is becoming less virulent;
3. that environmental factors are influencing the virulence of the virus