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Thousands of Seattle residents who had flu-like symptoms in early 2020 likely contracted the coronavirus, according to a new study published in EClinicalMedicine, an online open access journal for The Lancet.
The early spread of COVID-19 in Seattle was more extensive than first reported, the study authors wrote, with an estimated 6,500 undetected cases of COVID-19 between Feb. 24 and March 9.
The research team analyzed data from recent studies in Seattle and Wuhan, China, that re-tested samples from patients who thought they had the flu. Then the team estimated the ratio of COVID-19 to influenza infections.
In Wuhan, for instance, there were 4 coronavirus positives and 7 flu positives out of 26 patients who went to two hospitals for a flu-like illness between Dec. 30 and Jan. 12. In Seattle, there were 25 coronavirus positives and 442 influenza positives out of 2,054 adults and 299 children who reported respiratory illnesses before March 9, when the city imposed lockdown restrictions.
In Seattle, that means there was 1 COVID-19 case for every 7 flu cases among adults and 1 COVID-19 cases for every 9 flu cases among kids. In Wuhan, that means 2 COVID-19 cases for every 3 flu cases.
The researchers expected to find undetected COVID-19 cases early in the pandemic, but the ratios were surprisingly high, they wrote. In Seattle, more than 2,200 children and more than 4,300 adults were likely infected between Feb. 24 and March 9, they estimated.=
At the time of each city’s lockdown, Wuhan had reported 422 cases, and Seattle had confirmed 245 cases. Wuhan may have had 10,000 undetected cases before its Jan. 23 lockdown, they estimated.
“The spread of COVID-19 in Wuhan and Seattle was for more extensive than initially reported,” the study authors wrote. “The virus likely spread for months in Wuhan before the lockdown.”
EClinicalMedicine, “Using the COVID-19 to influenza ratio to estimate early pandemic spread in Wuhan, China and Seattle, US.”