The early spread of COVID-19 in Seattle and the city of Wuhan, China, was far more extensive than initially reported, according to a new study.

Researchers analyzed data from studies that re-tested swabs from influenza-like illnesses and acute respiratory infections in early 2020. Although they were not surprised to find overlooked COVID-19 cases in both cities, the ratios of SARS-CoV-2 to influenza-positive swabs were surprisingly high, reported Lauren Ancel Meyers, Ph.D., of the University of Texas at Austin. 

In Seattle, there was one adult case of symptomatic COVID-19 per every seven influenza cases and one pediatric case of symptomatic COVID-19 per every nine influenza cases. In Wuhan, there were approximately two symptomatic cases of COVID-19 for every three cases of influenza. 

“Given that influenza was circulating widely at the time of these infections, these ratios led us to conclude that there may have been over 5,000 undetected cases of symptomatic COVID-19 both in Wuhan prior to Jan. 12 and in Seattle prior to March 9,” the researchers wrote.

The study was published in EClinicalMedicine, an online open access journal of The Lancet.