A new report from a WHO infectious disease modeling team based at Imperial College London is estimating about two-thirds of Covid-19 cases worldwide have gone undetected. The analysis suggests the global spread of the novel coronavirus is significantly greater than the current volume of confirmed cases.

Source: Imperial College London

“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,” explains Natsuko Imai, one of the authors on the new report. “Our analysis, which extends and confirms previously released analysis by other groups using flight volumes from Wuhan City and the reported number of COVID-19 cases, demonstrates the importance of surveillance and case detection if countries are to successfully contain the epidemic.”

As of February 24, 2020, there are over 79,000 confirmed cases of Covid-19, the vast majority of which are concentrated in China. Prior studies have suggested most cases present with mild symptoms. Only a small minority of confirmed cases seem to progress into serious pneumonia-like disease, leading some researchers to suggest the virus may have spread wider than current numbers indicate.

This new report set out to explore how accurate current country-based surveillance of the disease may be, in relation to the average volume of travelers flying out of the epidemic epicenter in Wuhan, China.

“We compared the average monthly number of passengers traveling from Wuhan to major international destinations with the number of COVID-19 cases that have been detected overseas,” says another author on the new report, Sangeeta Bhatia. “Based on these data, we then estimate the number of cases that are undetected globally and find that approximately two thirds of the cases might be undetected at this point. Our findings confirm similar analyses carried out by other groups.”

The report concludes it is very likely a number of undetected chains of transmission have begun in many countries across the globe. Director-General of the World Health Organization Tedros Adhanom Ghebreyesus affirmed this growing concern during a recent WHO briefing.

“Although the total number of Covid-19 cases outside China remains relatively small, we are concerned about the number of cases with no clear epidemiological link, such as travel history to China or contact with a confirmed case,” said the Director-General in the February 22^nd briefing.

Although the Director-General did state the “window of opportunity for containing this coronavirus is narrowing”, the WHO has not yet classified this outbreak as a pandemic. Less than 2,000 cases have been currently confirmed outside of China, however, as this new report indicates, the viral spread may be broader than the official numbers suggest.

Jennifer Nuzzo, an epidemiologist from Johns Hopkins University, said last week the spread of the virus already constitutes a global pandemic, despite the WHO’s reluctance to deem it so officially. While the difference between an epidemic and a pandemic may be a somewhat semantic one, based on geographic spread of the disease, Nuzzo suggests the terminology does fundamentally affect how we deal with the virus on a local level.

“I am of the belief … that we are in the early stages of a pandemic,” said Nuzzo. “I think it’s important for us to … talk openly about whether containment of an epidemic is possible, or whether we are exacerbating pandemic spread, because my worry is we’re diverting resources from the community level.”