COVID-19 Update: 17 June 2020

After a brief hiatus, I have returned to watching news coverage of coronavirus. It seems that the current debate appears to be hung up on the idea of what COVID-19 metric matters. In other words, which is more important – cases, hospitalizations, ICU admissions? There seems to be some general agreement that deaths matter.

This debate reminds me of the decades long back-and-forth about blood pressure. Was it the systolic (top number) or diastolic (bottom number) that better predicted heart attacks, stroke and mortality? This debate went on and on. To the general public the answer will come as no great surprise – both matter. So I think Medicine and Epidemiology have a decent track record of tying themselves into knots over the ‘answer’ when more than one thing can be true.

I started looking a bit more closely at case and mortality rates in Santa Monica in comparison to Los Angeles County as a whole. Los Angeles County has contributed by far and away the most cases and deaths to California’s total: 73,018 cases and 2,907 deaths. Santa Monica has had 340 cases and 22 deaths to date.

I am in the process of reading Hans Roslin’s book “Factfulness” (well worth the read) and in it he gives lots of great advice about numbers and our tendency to generalize. In it he says “single numbers on their own are misleading and should make you suspicious. Always look for comparisons. Ideally divide by something.” And, further, “look for differences within groups, look for similarities across groups and do not assume that what applies for one group applies for another.”

Readers should be familiar now with my graphs of case rates which are presented as a daily number (i.e. number of cases per day) expressed as a function of 100,000 population. This daily rate is an average, smoothed over the prior 7 days to account for delays in reporting (and that fewer cases are identified over weekends, for example). So looking at Figure 1 we see that Santa Monica consistently has had fewer cases adjusted for population, and for the week ending 6/12/2020 we have about 1/3 as many as Los Angeles County as a whole: 3.97 daily cases versus 11.52 daily cases.

Good news right?

Seems to be until you compare average daily mortality rates – these are shown in Figure 2. For the week ending 6/12/2020, Santa Monica and Los Angeles County have essentially identical rates: 0.26 daily deaths for Los Angeles County and 0.27 for Santa Monica.

So what does this mean?

Well to return to Rosling’s basic tenets of Factfulness, Los Angeles County and Santa Monica have very different case rates. However, these two regions have essentially identical mortality rates. Said in another way, 44 cases in Los Angeles County are necessary to result in one death but only 14 cases are needed to lead to one death in Santa Monica.

Now I’m obviously not the first person to make this observation (although to be fair I have not seen this phenomenon expressed so bluntly). Johns Hopkins Coronavirus Resource Center has noted such differences in mortality rates across countries and suggests a few explanations. These include:

  1. Differences in the number of people tested: With more testing, more people with milder cases are identified. This lowers the case-fatality ratio (possible but daily testing rates for Santa Monica are not published).
  2. Demographics: For example, mortality tends to be higher in older populations (possible but daily counts by age are not published by Los Angeles County or Santa Monica).
  3. Characteristics of the healthcare system: For example, mortality may rise as hospitals become overwhelmed and have fewer resources (theoretically possible but Santa Monica hospitals have certainly not been overwhelmed by COVID-19 cases).
  4. Other factors, many of which remain unknown (a particularly unsatisfying explanation).

But I think one key point can be concluded from this analysis. Both cases and deaths matter. But, if it takes fewer cases in a particular community to result to a single death, then reduction in cases takes on a heightened urgency.

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