re: Covid 19: misunderstandings in statistics, ascertainment bias (more testing= more "cases")

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If COVID-19 Killed 1.4% of People With Symptoms in Wuhan, the Overall Fatality Rate Is Likely to Be Much Lower Than People Feared The big unknown is how many people are infected but aren't counted in the official numbers because their symptoms are mild or nonexistent.

JACOB SULLUM | 3.20.2020 2:10 PM

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A new study of COVID-19 cases in Wuhan, China, estimates that the death rate among people who were infected and developed symptoms was 1.4 percent. That is far lower than the crude case fatality rate (CFR) produced by dividing total deaths into total confirmed cases (4.5 percent) and far lower than the global CFR initially calculated by the World Health Organization (3.4 percent). The study, reported yesterday in Nature Medicine, suggests that the overall CFR—including people who are infected but do not develop symptoms—will prove to be much lower in the United States than many people feared.

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That's a very wide range, highlighting the weak empirical basis for aggressive, economically ruinous COVID-19 control measures. If the CFR is as low as 0.05 percent, which is half the estimated CFR for the seasonal flu, the number of deaths in the CDC's worst-case scenario would be 107,000, meaning the projection is off by a factor of 16. If a third of the population is ultimately infected, rather than the 65 percent assumed in the CDC's worst-case scenario, the number of deaths plummets from 1.7 million to about 55,000. It is impossible to assess the cost-effectiveness of mass interventions such as statewide or nationwide "shelter in place" orders in the face of such uncertainty.

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The study's main symptomatic CFR calculation assumes that half of the people infected by the COVID-19 virus develop symptoms. If that's true, the number of infections is at least twice as high as the number of known cases, and that's assuming everyone with symptoms gets counted, which is certainly not the case, since people with mild symptoms may never seek medical attention or testing. But that factor alone means that the overall CFR is dramatically lower than it appears to be based on reported cases and deaths. "One largely unknown factor at present is the number of asymptomatic, undiagnosed infections," the researchers note. "Estimates of both the observed and unobserved infections are essential for informing the development and evaluation of public health strategies, which need to be traded off against economic, social and personal freedom costs. For example, drastic social distancing and mobility restrictions, such as school closures and travel advisories/bans, should only be considered if an accurate estimation of case fatality risk warrants these interventions, which seriously disrupt social and economic stability."

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