Coronavirus Was Able to Spread so Fast Because of 'Stealth Transmissions,' Study Finds

The rapid spread of the COVID-19 outbreak in China can largely be explained by undetected "stealth transmissions"—or cases in which symptoms were not severe, according to a study.

Scientists from Columbia University's Mailman School of Public Health used advanced computer simulations to mathematically model the spread of the novel coronavirus for a paper published in the journal Science.

The novel coronavirus, known as SARS-CoV-2, emerged in Wuhan, China, toward the end of 2019 and rapidly spread across the country and, subsequently, internationally.

"Efforts to contain the virus are ongoing; however, given the many uncertainties regarding pathogen transmissibility and virulence, the effectiveness of these efforts is unknown," the researchers wrote in the paper. "The fraction of undocumented but infectious cases is a critical epidemiological characteristic that modulates the pandemic potential of an emergent respiratory virus."

"These undocumented infections often experience mild, limited or no symptoms and hence go unrecognized, and, depending on their contagiousness and numbers, can expose a far greater portion of the population to virus than would otherwise occur," the authors said.

Using their models, the scientists estimated the contagiousness and proportion of undocumented infections in China during the weeks before and after the shut down of travel in and out of Wuhan on January 23. This kind of research is critical for understanding the overall prevalence of the COVID-19 disease.

According to the researchers, 86 percent of all infections were undocumented before the Wuhan travel shutdown. Per person, these undocumented cases were around half as contagious as documented cases. However, they were the source of two-thirds of all documented infections. These findings can help to explain the rapid spread of the virus and indicate that it will be particularly difficult to contain.

"The explosion of COVID-19 cases in China was largely driven by individuals with mild, limited, or no symptoms who went undetected," Jeffrey Shaman, a co-author of the study from the 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," he said.

Nevertheless, the researchers say that the efforts of the Chinese government appeared to slow the spread of the virus.

"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," Shaman said. "If the novel coronavirus follows the pattern of 2009 H1N1 pandemic influenza, it will also spread globally and become a fifth endemic coronavirus within the human population."

"A rough rule of thumb is that about half of the world population is infected by an influenza pandemic in two years. If COVID-19 follows suit, that's about 3.5 billion infections, 500 million confirmed cases, and 11.5 million deaths. This is an upper bound and assumes no advances in the point-of-care treatment of confirmed cases, no new therapeutics, and no vaccine. An effective vaccine or new therapeutic would greatly reduce mortality," he said in a statement.