Thousands of early cases went undetected
Thousands of COVID-19 cases in the early stages of the global pandemic may have gone undetected in two of the initial epicenters for the virus, according to a new study by University of TexasAustin researchers.
The new report, published in a scientific journal Tuesday, estimated that COVID-19 was already spreading in Wuhan, China, and Seattle, Washington weeks ahead of lockdown measures in each city. The report used data from two separate studies in those cities where researchers retested throat and nasal swabs taken from patients who were suffering from flu-like symptoms during January in Wuhan and late February and early March in Seattle.
A team of researchers at UT-Austin, led by Lauren Ancel Meyers, a professor of integrative biology and statistics and data sciences, used that data to create an epidemiological model to show the extent of early COVID-19 spread in Wuhan and Seattle. Meyers and her team found that in Wuhan, there may have been more than 12,000 undetected symptomatic cases of COVID-19 prior to the city’s Jan. 23 lockdown. On March 9 in Seattle, the week when schools closed due to the virus, Meyers estimates more than 9,000 people had undetected COVID-19, with children making up one-third of that total.
“As the virus started spreading in cities across the United States and cities across the world, probably by the time we started detecting cases, it’s likely that the virus had been spreading for several weeks, if not months,” Meyers said.
These new estimates reinforce how quickly and stealthily the virus spread upon reaching U.S. soil, transmitting at a rate too fast for most cities to ramp up testing capacity.
In Texas, Meyers estimates the virus started spreading in cities such as Austin as far back as midFebruary, weeks before the city’s first reported case on March 13. The Houston Rodeo began March 3 and shut down on March 11, after a Montgomery County man who had attended the rodeo fell ill and tested positive. As of Wednesday, Texas had surpassed 500,000 confirmed COVID-19 cases, days before schools will begin to reopen across the state.
Meyers’ models were extrapolated directly from the retested throat and nasal swabs in Wuhan and Seattle. Her team used the ratio of swabs that tested positive for COVID-19 compared to the flu-positive swabs, along with surveillance data for the flu across China and Washington, to arrive at an estimate for how many undetected coronavirus cases there were in these cities in the weeks before they shut down.
The Wuhan study tested 26 throat swabs taken from adults over 30 who sought care for flulike symptoms at two central city hospitals between Dec. 30 and Jan. 12. Of the retested swabs, four tested positive for COVID-19, while seven tested positive for flu.
In Seattle, researchers tested over 2,353 nasal swabs collected from 299 children under 18 and 2,054 adults who reported flu-like symptoms between Jan. 1 and March 9. Of these samples, 442 tested positive for flu, while 25 tested positive for COVID-19.
Using this data, Meyers concluded that in Wuhan, for every three cases of flu in adults there were two undetected cases of COVID-19, meaning there were almost 13,000 cases of symptomatic COVID-19 prior to the city’s Jan. 23 lockdown. The virus probably started spreading in Wuhan as far back as mid-November, Meyers said.
In Seattle, the COVID-19-flu ratio was lower, with one case of undetected COVID-19 for every nine cases of detected flu in children and a one-to-seven ratio in adults. The data suggest that the virus started spreading in Seattle in early January. On March 9, Seattle had only 245 reported COVID-19 cases, but Meyers said the data indicate there could have been as many as 9,000 symptomatic cases.
“If there really were 9,000 cases or so in Seattle at a time when 245 cases had been reported, that should corroborate that there’s a lot of transmission that went on back then and probably still goes on today that is undetected, that is undocumented,” Meyers said, surmising that part of the reason could be that people without symptoms don’t bother to get tested for the virus.
Meyers noted, however, that her team’s estimates do not account for COVID-19 cases where the infected person has no symptoms. Given that that asymptomatic spread of the virus has contributed heavily to the pandemic, Meyers estimated that there may have been as many as 20,000 COVID-19 cases in Wuhan when factoring in asymptomatic people, and as many as 15,000 cases in Seattle.
But the fundamental goal of the study, Meyers said, is to use available data to understand as much as they can about the virus. She hopes to eventually do similar modeling for Texas cities once the types of data used in Wuhan and Seattle becomes available.
“We are using our models to do a number of different things, but one is just trying to connect the dots: to understand how the virus spreads, how it emerged, what it was doing early in the pandemic, so we can get a better handle on what’s happening today and make better projections in the future.”