Undocumented COVID-19 Cases Responsible for Majority of Spread, Study Finds


New research from an international team of investigators suggests more than 80% of cases have been transmitted by undocumented infections.


New research from Columbia University is shedding light on how documented and undocumented cases of COVID-19 have contributed to the ongoing and rapid spread of the ongoing pandemic.

Through observations reported within China and the use of an advanced computer model, an international team of investigators uncovered data suggesting undocumented COVID-19 cases were responsible for more than 80% of the infections that occurred.

“Depending on their contagiousness and numbers, undetected cases can expose a far greater portion of the population to virus than would otherwise occur,” said investigator Jeffrey Shaman, PhD, professor of environmental health sciences at Columbia University Mailman School, in a statement. “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.”

In an effort to better understand the potential scope of the outbreak and possibly decrease the spread of COVID-19, Shaman and colleagues from London and Beijing developed an advanced computer model to estimate contagiousness and proportion of undocumented infections in China before and after the shutdown of travel in and out of Wuhan. The mathematical model incorporated data from infections across 375 Chinese cities and divided infections based on whether they were documented or observed and whether they were undocumented infections.

To calculate human mobility before China imposed travel restrictions, investigators performed an analysis using mobility data from January 10-23 and again from January 24-February 8, which was used to estimate changes following the travel restrictions. Investigators also included a number of control measures in the analysis, including travel restrictions between major cities, self-quarantine, and increased availability of testing.

While multiples models were run by investigators, they determined the “best-fitting” model indicated 13,118 total new COVID-19 infections—both documented and undocumented—occurred between January 10-23 in Wuhan. Of these, 86.2% (95% CI, 81.5%— 89.8%) had become infected from undocumented cases. Across China, the total number of infections during that time period as 16,829 (95% CI, 3797–30,271) and, similarly, 86.2% of the infections were caused by undocumented cases.

When analyzing the impact of travel restrictions, which included a 98% reduction in traffic to and from Wuhan and an 80% reduction in other intercity travel, and other measures in the post-January 23 period, investigators found the transmission rate of infections decreased by almost half from the prior period (0.52; 95% CI, 0.39—0.71 vs. 0.35; 95% CI, 0.27–0.50).

Investigators noted the apparent findings indicate extreme increases in isolation and identification could lead to significant improvements in control of SARS-CoV2 but cautioned they may not completely end the spread of the outbreak.

“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, in the aforementioned statement. “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.”

This study, titled “Substantial undocumented infection facilitates the rapid dissemination of novel coronavirus (SARS-CoV2),” is published online in Science.

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