Epidemiology, News, ProMED

COVID-19: What is the Denominator

Author Dr. Stephen Berger , 29-02-2020

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Since the 1st cases of infection by SARS-CoV-2 were reported in China, we have all been confronted by death and case-fatality statistics, which are both misleading and inaccurate. As of this morning, 2837 from a total of 83 774 reported cases of COVID-19 were fatal. Public Health professionals, the lay public, and politicians will conclude that this disease carries a “mortality rate” of 3.4%. Relatively few realize that “only” 1.4% of patients treated outside of Mainland China have died of COVID-19: 0.7% of passengers on the Diamond Princess cruise ship, 0.5% of patients in South Korea, etc.

One explanation for these discrepant case-fatality statistics is related to demography. Patients reported by official sources have a higher mean age and prevalence of underlying chronic disease than the general Chinese population (or international travelers). Case-definition, variation in quality of care, genetic and nutritional factors might also explain higher fatality rates among Chinese patients. Indeed, several of the patients who died of COVID-19 outside of Mainland China have also been Chinese Nationals.

A fundamental error in all of this could be related to the term, “reported cases.” How many infections in China are asymptomatic or sub-clinical? If, for example, only one-in-10 individuals who acquire infection by SARS-CoV-2 are sufficiently ill to visit a clinic or hospital, the true case-fatality rate decreases from 3.4% to 0.34% A seroprevalence survey in the general population could easily determine the true impact of this disease.

This week, details of an antibody assay for SARS-CoV-2 infection were reported in Journal of Medical Virology. [1] The authors state that the procedure detects both IgM and IgG antibodies within 15 minutes, with a test sensitivity and specificity of 88.86% and 90.63%, respectively.

Beyond the search for vaccines and effective anti-viral agents, an immediate remedy for much of the uncertainty regarding COVID-19 will be a comprehensive serological survey of populations in affected areas. Tested individuals should be questioned regarding recent travel, occupational contact, and relevant symptoms which may have occurred during the preceding 2 months.

If a high background sero-prevalence rate exists among people in Wuhan (or China as a whole) COVID-19 becomes little more threatening than other “new strains of flu” that we deal with each year.

1. Li S, Yi Y, Lou X, et al. 2020. Development and Clinical Application of a Rapid IgM‐IgG Combined Antibody Test for SARS‐CoV‐2 Infection Diagnosis. J Med Virol (ahead of print) PMID: 32104917 DOI: 10.1002/jmv.25727.


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Dr. Stephen Berger

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