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The difference implies that the virus is much less deadly than it looks, but it also makes contact tracing a daunting challenge.

Newly published antibody test results from half a dozen parts of the country confirm that COVID-19 infections in the United States far outnumber confirmed cases. The ratio of estimated infections to known cases in these studies, which the U.S. Centers for Disease Control and Prevention (CDC) reported on Friday, range from 6 to 1 in Connecticut as of early May to 24 to 1 in Missouri as of late April.

These results confirm something we already knew: The COVID-19 infection fatality rate—deaths as a share of all infections—is much lower than the crude case fatality rate—deaths as a share of known cases. That is bound to be true when testing is limited and a virus typically produces mild or no symptoms. At the same time, the CDC’s antibody studies imply that efforts to control the epidemic through testing, isolation, quarantine, and contact tracing will not be very effective, since they reach only a small percentage of virus carriers.

The CDC analyzed blood samples drawn for routine tests unrelated to COVID-19 from patients in New York City, Connecticut, South Florida, Missouri, Utah, and western Washington state. Although these samples may not be representative of the general population, they provide a clearer picture of virus prevalence than screening limited to people who sought virus tests because they had symptoms consistent with COVID-19 or because they were in close contact with known carriers.



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