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COVID infection counts are meaningless unless you know the context in which the counts were done: they underestimate actual cases by a factor of two to a hundred: 538 by Nate Silver

2020-04-13

(image courtesy of pixabay.com and Gerd Altmann)

A report on the blog “FiveThirtyEight” from April 4, written by Nate Silver (already famous for his political polling expertise), explains why the case counts for COVID-19, the disease caused by SARS-COV-2, vary so dramatically from country to country.  More importantly, they explain why all case counts deeply underestimate the actual number of infections– by a rate of anywhere from double to a hundred times less.

The blog post is long and complex, but those of you with a statistical bent or who are comfortable with numbers will find it fascinating.  It explains what goes into a case count and why those numbers lag behind actual cases anywhere from a few days to two weeks.  There are many reasons, from lack of testing materials to political propaganda (in some cases), for this undercount.

Attempts to determine the final case fatality rate (CFR, the number of patients with infection who eventually die) will not be successful until the peak of the pandemic has long passed.  It takes, usually, about five days (anywhere from two days to more than two weeks) from exposure to the time symptoms appear (if they appear) or an infection is established.  Nasopharyngeal swab testing for the presence of viral RNA (the actual genome of the virus) is most likely to show positive results by day eight of infection, and may precede symptoms by up to 48 hours– that is, if symptoms ever appear.  Tests at this time show about 75% of actual cases.  Once the infection is established, it may take anywhere from 48 hours to three weeks before a patient succumbs (dies) from the virus.  Not all deaths are  recorded as being due to the virus: for example, in New York in late March, there were over five thousand excess deaths, but only half of these deaths were recorded as being due to the novel coronavirus.  Many, if not most, sudden deaths at that time were assumed to be due to stroke or heart attack; if they occurred at home before medical care was sought, there may be a mystery as to what symptoms preceded the demise (death).

A single excess death, viewed alone, cannot not be ascribed to the virus unless testing is done.  Evaluation of people already dead seemed like a waste of resources in light of the shortage of tests, but it impaired our ability to see the excess deaths as due to the virus.  People have been found dead at home, in the subway, lying on the sidewalk under a pile of newspapers, and in other unlikely places.  Handling people recently deceased from the virus could be hazardous to the health of first responders who are needed for many other things during this emergency.

Another issue which is never mentioned (except in Nate Silver’s blog post) is the false positive rate (the rate of positive tests when the patient does not actually have the disease).  This rate is trivial for the RT-PCR test except when the total positive rate is very small.  In cases of very rare disease, the false positive rate, even if very low, may overwhelm the true positive rate to such an extent that confirmatory tests are needed to establish actual disease.  In such cases, with very rare diseases, a positive test must be considered a “suspected” case and lead to further investigation.  False positives may overestimate disease prevalence (the rate of disease at any point in time) by as much as two hundred to one in cases of rare diseases such as acromegaly (abnormal bone enlargement due to high levels of growth hormone) and autoimmune disorders.

All these factors (excluding false positive rates, which are trivial in such a common disease) have led to an underestimation of the case counts and deaths related to SARS-COV-2.  Despite these obvious factors, the conservative media has been pushing the conspiracy theory that deaths due to the virus have been grossly overestimated.  This is obviously false, but it feeds into the conservative mind-set that believes that the whole pandemic is a Democratic hoax.  Nothing could be further from the truth or more dangerous at this dangerous time.

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