There were thousands of undetected early cases of COVID-19 at the beginning of this year, according to a study which found that the disease was far more widespread in Wuhan, China, and Seattle in the US weeks ahead of lockdown measures in each city. Researchers from The University of Texas (UT) at Austin in the US also concluded that the first case of COVID-19 in Seattle may have arrived as far back as Christmas or New Year's Day.
The study, published in the journal EClinicalMedicine, also found that about a third of the estimated undiagnosed cases in the US were among children. The researchers extrapolated 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 analysed 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, the researchers said. "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," said Lauren Ancel Meyers, a professor at UT Austin.
"Since we knew how widespread flu was at that time, we could reasonably determine the prevalence of COVID-19," Meyers said. When the Chinese government locked down Wuhan on January 22, there were 422 known cases, the researchers said.
However, extrapolating the throat-swab data across the city using a new epidemiological model, Meyers and her team found that there could have been more than 12,000 undetected symptomatic cases of COVID-19. On March 9, the week when Seattle schools closed due to the virus, researchers estimate that more than 9,000 people with flu-like symptoms had COVID-19 and that about a third of that total were children.
The data do not imply that health authorities were aware of these infections, rather that they may have gone unseen during the early and uncertain stages of the pandemic. "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," said Zhanwei Du, a postdoctoral researcher in Meyers' lab and first author on the study.
According to several other studies, about half of COVID-19 cases are asymptomatic, leading researchers to believe that there may have been thousands more infected people in Wuhan and Seattle before each city's respective lockdown measures went into effect. "We can go back and piece together the history of this pandemic using a combination of investigative techniques and modeling," Meyers said.
"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," she said.