Patients with undiagnosed flu symptoms who actually had COVID-19 last winter were among thousands of undetected early cases of the disease at the beginning of this year. In a new paper in The Lancet's open-access journal EClinicalMedicine, epidemiological researchers from The University of Texas at Austin estimated COVID-19 to be far more widespread in Wuhan, China, and Seattle, Washington, weeks ahead of lockdown measures in each city.

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In the U.S., about a third of the estimated undiagnosed cases were among children. The researchers also concluded that the first case of COVID-19 in Seattle may have arrived as far back as Christmas or New Year's Day.

Lauren Ancel Meyers, a professor of integrative biology and statistics and data sciences who leads the UT Austin COVID-19 Modeling Consortium, worked with her team of researchers to extrapolate the extent of the COVID-19 epidemic in Wuhan and Seattle based on retested throat swabs taken from patients who were suffering from influenza-like illnesses during January in Wuhan and during late February and early March in Seattle. When the samples were analyzed later in each city, most turned out to be flu, but some turned out to be positive for SARS-CoV-2, the virus that causes COVID-19.