Researchers have found that there were thousands of undetected early cases of COVID-19 at the beginning of this year and the disease was far more widespread in Wuhan, China, and Seattle in the US, weeks ahead of lockdown measures in each city.

For our comprehensive coverage and latest updates on COVID-19 click here.

The study, published in the journal EClinicalMedicine, also concluded that the first case of COVID-19 in Seattle may have arrived as far back as Christmas or New Year's Day. "Even before we realised that COVID-19 was spreading, the data implies that there was at least one case of COVID-19 for every two cases of flu," said study researcher Lauren Ancel Meyers from the University of Texas at Austin in the US. "Since we knew how widespread flu was at that time, we could reasonably determine the prevalence of COVID-19," Meyers added. The research team 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 COVID-19 virus. When the Chinese government locked down Wuhan on January 22, there were 422 known cases. But, extrapolating the throat-swab data across the city using a new epidemiological model, the 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 study author Zhanwei Du. 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. "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," the study authors wrote.