Study: COVID-19, diagnosed as flu and spread more extensively, earlier than reported

Editor's Note

The spread of COVID-19 in Seattle and Wuhan, China, which coincided with the influenza season, was far more extensive than initially reported, and the virus had likely been spreading for several weeks before official records indicate, this study from the University of Texas at Austin finds.

Researchers examined two different periods—the month of January in Wuhan and the weeks of late February and early March in Seattle. Throat swabs from outpatients diagnosed with flu-like symptoms during these time periods were reanalyzed for the presence of COVID-19.

In both cities, most of the cases turned out to be the flu, but four of 26 retested samples tested positive for COVID-19 in Wuhan, and 25 of 2,353 retested samples tested positive for COVID-19 in Seattle.

Based on the trajectory of the flu season in both areas, the researchers created a model of how early and widespread COVID-19 had likely been during the first weeks.

They estimated that Seattle already had at least 9,000 cases by March 9, when the city implemented lockdown measures, while only 245 cases were actually reported. These thousands likely involved asymptomatic children or those with mild cases.

Wuhan similarly had more than 12,000 cases by January 22, as the Chinese government issued its lockdown, but they reported only 422 official cases.

The researchers also estimated that people were spreading COVID-19 in Seattle by the first week of January and as early as December 25, 2019, and that the first Wuhan case occurred sometime between late October and early December 2019.

The researchers say their method also can be used to estimate how many COVID-19 cases were mistaken for flu in other areas of the US hit early by the pandemic, such as New York City and California.