New research suggests the rapid spread of SARS-CoV-2, the virus responsible for COVID-19, in China was primarily due to “stealth transmission” as people with mild or non-existent symptoms continued to move and unknowingly spread the disease.
The work, published in Science, uses information based on the documented infections in China between January 10 and 23 as well as after the travel ban between January 24 and February 8 to estimate how “stealth transmission” affected infection rates.
The model suggests that 86 percent of all infections were undocumented prior to January 23, a value that matches estimations based on the infected foreign nationals that were evacuated from Wuhan. The model suggests that, on average, these undocumented infections were slightly more than half as contagious as documented infections but still made up two-thirds of documented infections.
"The explosion of COVID-19 cases in China was largely driven by individuals with mild, limited, or no symptoms who went undetected," co-author Professor Jeffrey Shaman, from the Columbia University Mailman School, said in a statement. "Depending on their contagiousness and numbers, undetected cases can expose a far greater portion of the population to virus than would otherwise occur. We find for COVID-19 in China these undetected infected individuals are numerous and contagious. These stealth transmissions will continue to present a major challenge to the containment of this outbreak going forward."
The researchers warn of limitations to the modeling, since it is based on “undocumented infections” that by definition we are not entirely aware of. The team also points out that care-seeking behavior and government control measures were not binary between the two periods and were in flux, so the model simplifies how the transmission played out.
"Heightened awareness of the outbreak, increased use of personal protective measures, and travel restriction have helped reduce the overall force of infection; however, it is unclear whether this reduction will be sufficient to fully stem the virus spread," added Shaman.
Social distancing and good hygiene (washing your hands for 20 seconds with soap and water) are both important steps to help to reduce the spread of the virus.