An analysis based on re-tested throat swabs from people in Wuhan, China, and Seattle, United States, suggests that thousands of people who had flu-like symptoms last winter could actually have had COVID-19.

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A new analysis suggests that COVID-19 may have been present in thousands of people with flu-like symptoms in winter 2019.

All data and statistics are based on publicly available data at the time of publication. Some information may be out of date. Visit our coronavirus hub for the most recent information on COVID-19.

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On December 31, 2019, China announced a cluster of pneumonia cases in Wuhan, which later transpired to be the new coronavirus disease (COVID-19).

The U.S. reported its first case of COVID-19 just 21 days later, on January 20, 2020. The person in question was a 35-year-old man in Washington who had returned to the U.S. 5 days earlier after visiting family in Wuhan.

Understanding how the SARS-CoV-2 virus spread in the early stages of the pandemic is important for a number of reasons, including accurately determining the fatality rate of COVID-19.

A study led by The University of Texas at Austin has applied a new method to estimate the early spread of COVID-19, which is based on the ratio of COVID-19 to influenza cases.

The researchers estimate that there were many undiagnosed COVID-19 cases in Wuhan and Seattle, WA, even before the implementation of lockdowns in these cities.

The analysis, which is available in the Lancet journal EClinicalMedicine, suggests that the early spread of COVID-19 may have been far greater than experts initially thought.

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The analysis is based on two previously published studies — one from Wuhan in China and one from Seattle in Washington, the state with the first reported case of COVID-19. These studies re-tested swabs taken from people with flu-like symptoms during the time when COVID-19 may have been silently spreading.

The Wuhan study identified four people with a SARS-CoV-2 infection out of 26 people aged 30 and over who sought care for flu-like symptoms before January 12, 2020.

The Seattle study found 25 people who were positive for COVID-19 out of 2,353 people who reported respiratory illness before March 9, 2020.

People in both studies also tested positive for influenza. Among adults in Wuhan, there were roughly two cases of COVID-19 for every three cases of flu, while in Seattle, there was roughly one case of COVID-19 for every 18 cases of flu.

Using this data, the team behind the new study calculated the ratio of COVID-19 to influenza infections. Using this in combination with data on the overall prevalence of flu, they were able to estimate the number of (symptomatic) COVID-19 infections in this period.

“We can go back and piece together the history of this pandemic using a combination of investigative techniques and modeling,” explains Prof. Lauren Ancel Meyers, senior author of the new study.

“This helps us understand how the pandemic spread so quickly around the globe and provides insight into what we may see in the coming weeks and months.”

When the government locked down Wuhan on January 23, there were 422 known cases. However, the new study estimates that even before this lockdown was announced (between December 30, 2019, and January 12, 2020), there were already more than 1,300 cases of COVID-19 in Wuhan.

“Even before we realized that COVID-19 was spreading, the data imply that there was at least one case of COVID-19 for every two cases of flu,” says Prof. Meyers.

In Seattle, the researchers estimate that 4,367 adults had symptomatic COVID-19 between February 24 and March 9, 2020. They also estimate that 2,268 children had the infection in this period, which was prior to schools closing in the region.

“Given that COVID-19 appears to be overwhelmingly mild in children, our high estimate for symptomatic pediatric cases in Seattle suggests that there may have been thousands more mild cases at the time,” the authors write.

On the basis of these findings, the researchers say that the pandemic likely started from a single person in Wuhan, who may have developed symptoms as early as the end of October 2019.

They suggest that the transfer of the disease to Seattle probably occurred between December 25, 2019, and January 15, 2020.

Overall, the study findings suggest that the early spread of COVID-19 — at least in these two cities — was far greater than the authorities initially reported. The disease likely spread for months before the introduction of lockdowns.

Furthermore, the study only estimates symptomatic cases of COVID-19. As people can carry the SARS-CoV-2 virus without having any symptoms, the actual number of pre-lockdown cases may be much higher.