Two-Thirds of Virus Carriers From China Weren’t Detected by Countries, Study Estimates

A man (L) disembarking from a Xiamen Airlines plane at the Tianhe airport in Wuhan, Hubei Province, China, on Jan. 31, 2020. STR/AFP via Getty Images

A new study estimates that two-thirds of coronavirus cases exported from mainland China remain undetected worldwide.

In less than two months, the virus has spread to 29 countries and territories outside of mainland China, with a total of more than 1,000 confirmed cases.

However, a Feb. 21 study (pdf) by researchers at the Imperial College London, which collaborates with the World Health Organization (WHO) in infectious disease modeling, analyzed flight data from the virus epicenter Wuhan City, and found that “some countries have detected significantly fewer than would have been expected based on the volume of flight passengers arriving from Wuhan.”

Researchers found that countries including Singapore, Finland, Nepal, Belgium, Sweden, India, Sri Lanka, and Canada have been more effective in their virus detection efforts. After taking into account these countries’ air traffic from China, they appear to have detected relatively more cases of the virus carried by people from mainland China, the researchers said.

Using Singapore as a benchmark, the researchers estimated that there were likely 426 cases of the virus being exported from mainland China—as opposed to local cases of human-to-human contagion—leading them to conclude that “worldwide, two-thirds of COVID-19 cases exported from mainland China have not been picked up by surveillance programs,” study co-author Christl Donnelly said in a press release. Globally, the current confirmed total of virus cases exported from mainland China is 156.

This potentially leaves sources of human-to-human transmission around the world unchecked, the study concluded.

“We are starting to see more cases reported from countries and regions outside mainland China with no known travel history or link to Wuhan City,“ co-author Natsuko Imai said. ”Our analysis ... demonstrates the importance of surveillance and case detection if countries are to successfully contain the epidemic.”

Chinese authorities locked down Wuhan on Jan. 23, when many experts said it was too late to stem the spread of the disease. Before then, 5 million residents had already left the city, serving as potential carriers of the virus to other cities and around the world.

Since then, dozens of countries have enacted travel restrictions and measures to screen for the disease among passengers arriving from mainland China.

The United States has barred foreign nationals who have visited China for the past 14 days from entering the country. Screening and quarantine measures apply to U.S. citizens, residents, and their family members who have been in China within 14 days.

Outside of China, South Korea has the most confirmed infections with 204 cases. In the United States, 34 patients have tested positive for the virus.