Two thirds of Covid-19 cases exported from mainland China may be undetected

A new report by Imperial scientists estimates that two thirds of Covid-19 cases exported from mainland China may be undetected.

The Imperial scientists, based at the WHO Collaborating Centre for Infectious Disease Modelling within the MRC Centre for Global Infectious Disease Analysis , and J-IDEA - Abdul Latif Jameel Institute for Disease and Emergency Analytics , released their 6th report on the COVID-19 outbreak on 21 February.

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 Dr Sangeeta Bhatia Study author


Since the emergence of COVID-19 in December 2019, the team have adopted a policy of immediately sharing research findings on the developing pandemic. Their new report presents an assessment of the relative sensitivity of international surveillance of COVID-19.

Dr Sangeeta Bhatia , report author, explained: “We compared the average monthly number of passengers travelling from Wuhan to major international destinations with the number of COVID-19 cases that have been detected overseas. 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.”

As of 20 February 2020, over 74,000 cases of COVID-19 (formerly 2019-nCoV) have been reported in China (with 2121 deaths). Over 1000 cases have been confirmed in 29 regions and countries outside mainland China (including Hong Kong SAR and Macau SAR).

Increasing cases outside mainland China

In this report the team analysed COVID-19 cases reported and confirmed in different countries that were exported from mainland China, and estimated the number of cases that have remained undetected.

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. Dr Natsuko Imai Study author


Dr Natsuko Imai , report author, said: “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. 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."

Exported cases vary in the severity of their clinical symptoms, making some cases more difficult to detect than others. Some countries have detected significantly fewer than would have been expected based on the volume of flight passengers arriving from Wuhan City, China.

Human-to-human transmission

The team estimated that two thirds of COVID-19 cases exported from mainland China have remained undetected worldwide, potentially leaving sources of human-to-human transmission unchecked.

We would like to thank countries for their continued transparency in presenting information on new cases Gina Cuomo-Dannenburg Study author


Gina Cuomo-Dannenburg, report author, added: “We compiled data from a variety of publicly available sources, such as national and provincial ministries of health and local news, to determine information about travel history, exposure and symptom onset of individual patients observed outside of mainland China. We would like to thank countries for their continued transparency in presenting information on new cases, and would like to encourage communication of patient outcomes going forward to present a true picture of severity and clinical presentation.”

Based on comparisons with Singapore only, 63% of cases are estimated to be undetected. Comparing with Singapore, Finland, Nepal, Belgium, Sweden, India, Sri Lanka, and Canada, 73% of cases are estimated to be undetected.

Professor Christl Donnelly , report author, explained: “We were able to analyse data collected around the world to determine how individual countries compared to those who have reported and confirmed the high numbers of cases compared to their connectedness with Wuhan city (as judged by the average number of international passengers per month). Bringing these numbers together we have estimated that worldwide two thirds of COVID-19 cases exported from mainland China have not been picked up by surveillance programmes.”

Professor Neil Ferguson , report author, added:“Countries around the world have been focused on containing the international spread of COVID-19 by detecting cases in travellers from China. However, surveillance approaches and capabilities have varied between countries. The resulting variation in the numbers of cases detected in each allows us to assess what proportion of cases in travellers may have been detected overall, by comparing case numbers in the countries with the highest detection rates with those in other countries. In agreement with the results from similar studies, we conclude that approximately two-thirds of cases in travellers from China have not yet been detected. It is highly likely that some of these undetected cases will have started chains of transmission within the countries they entered.”
Photos and graphics subject to third party copyright used with permission or Imperial College London.

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  • Professor Christl A Donnelly FRS
    School of Public Health
  • Dr Sangeeta Bhatia
    School of Public Health
  • Dr Natsuko Imai
    School of Public Health

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  • MRC Centre for Global Infectious Disease Analysis - COVID-19 reports
  • J-IDEA - Abdul Latif Jameel Institute for Disease and Emergency Analytics

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