Numerators are necessary, and nimble; denominators are difficult. In any epidemiological assessment, the devil is in the details; and, more commonly, the devil is in the denominator. The phase, derived from a German proverb—“Der liebe Gott steckt im detail (God is in the detail)"—is especially relevant to COVID-19. Let me explain….
With the emergence of a new and worrisome pathogen, the medical and public health community responds initially to the notable cases, and rightly so. Our attention is focused on severe and unexpected events, which present as something out of the ordinary. With COVID-19, these were the cases in Wuhan with severe respiratory consequences. These initial cases, some resulting in death, immediately became the numerator. The denominator, from which we could describe relevant epidemiological characteristics such as case-fatality rates (CFR), was the total number of confirmed cases based on positive COVID-19 tests. To this day, we continue to intently follow these numbers. My phone app this morning reports 107,442 confirmed cases and 3648 deaths, for a CFR of 3.2%. Herein lies the genesis of epidemiological misinterpretation with global consequences.
Because COVID-19 tests are a limited resource, the most appropriate deployment was to those individuals who were most likely to have COVID-19. In the U.S., our initial testing required significant symptomology and reasonable exposure (travel) history. Again, this was sound; however, it opened the door to ignoring those individuals with subclinical and minimally clinical symptoms. What we are learning is that there can be a significant COVID-19 burden across a very wide spectrum of clinical presentations. As a consequence, we can have wide-ranging estimates of CFR and rates of severe disease. As more and more testing becomes available, we will become smarter; our estimates of the impact will become more accurate. Unfortunately, we now have widespread seeding of this virus across the globe.
COVID-19 is unfolding in a relatively predictable manner. Initial severe cases attract attention and very comprehensive mitigation efforts. As a respiratory virus with an incubation period that is longer than transoceanic flights and symptomology that is manifold, complete detection at checkpoints is impossible, allowing for widespread dispersion. Arrival in new populations is unchecked by existing immunity and, with a respectable basic reproduction number (the number of secondary cases generated by each case), easily seeds and spreads through communities. As this is occurring during the typical “respiratory virus season” in the Northern Hemisphere, COVID-19 cases can easily hide among the plethora of other viral acute respiratory infections. Amidst this outbreak, we spend too much time on recrimination, as opposed to supporting those medical and public health professions who are frontline to our response efforts.
I suspect that containment of COVID-19 is out of the question. Response and mitigation efforts are our now most imminent responsibility. Clinicians need to keep abreast of reliable information, participate with their health communities for adequate planning and resourcing. Finally, messaging everywhere—on hand hygiene, respiratory hygiene, social distancing, appropriate use of PPE in medical settings, and self-isolation when ill—is our best defense.