Many people have raised concerns about the accuracy of COVID-19 data from China. In this report, we present strong evidence that China's reported COVID-19 caseload was undercounted by a factor of nearly 40. Based on officially reported cases in China in January 2020, the odds of the novel coronavirus appearing by January 22, 2020, in Japan, Thailand, South Korea, the United States, and Taiwan—as it did—would have been minuscule.
In this report—one of several from a RAND Corporation team examining the role of commercial air travel in the coronavirus disease 2019 (COVID-19) pandemic—we use our COVID-19 Air Traffic Visualization (CAT-V) tool to estimate the likely number of infections in China in early 2020. The tool combines COVID-19 case data from Johns Hopkins University with detailed air travel data from the International Air Transport Association.
From December 31, 2019, to January 22, 2020, China reported a daily average of 172 cases of COVID-19 among its residents. This number of confirmed cases was equivalent to just one per 8.2 million residents in the country per day. Using the detailed flight data over that same period of time, we determined that the five countries most at risk of importing COVID-19 from China were, in descending order of risk, Japan, Thailand, South Korea, the United States, and Taiwan.
But far fewer than 8.2 million passengers flew from China to the five countries over that 23-day period. Just more than 1 million passengers flew from China to Japan and Thailand each, while slightly more than 750,000 flew to South Korea, 500,000 flew to the United States, and fewer than 400,000 flew to Taiwan (as illustrated in the map below). Thus, all of these passengers from China totaled fewer than 3.7 million, for an expected COVID-19 exportation rate of less than one case to all five of these countries combined. However, COVID-19 cases were already being reported in all five countries during this time. This trend would be exceedingly unlikely given the low reported case count in China.
If there were an average of 172 total cases per day in China through January 22, 2020, the odds of Japan and Taiwan importing even one case by that date would be 9 percent each. The odds of Japan, Thailand, South Korea, the United States, and Taiwan all reporting cases would be only one in 1.3 million.
For even odds of COVID-19 cases appearing in all five countries by January 22, 2020, the average odds of a case appearing in each of these countries would have needed to be roughly 87 percent. To reach those odds, the actual case rate in China would have needed to be about 37 times higher than what was officially reported on that date—that is, 18,700 total infectious cases, as opposed to just the 503 total cases that China reported having on January 22, 2020.
This estimate is broadly consistent with research published in a January 2020 Imperial College London report and an April 2020 article in the Emerging Infectious Diseases journal, both of which estimated that there were about 4,000 cases in the city of Wuhan, Hubei province, on January 18 and 18,700 cases there on January 23.
In accordance with RAND's quality assurance standards, this analysis is based on the best available data. However, COVID-19 is an evolving threat, and even the best available data being used by government agencies and research institutes have very significant limitations. In the first report in this series, we outline several caveats about using country-level data, assuming equal passenger risk profiles, drawing on inaccurate country caseload reports, and being restricted by other data limitations.
For the analysis described in this report, it is important to note that we assume that the arrival of COVID-19 cases in the five destination countries was a result of air travel from China. No other sources of exportation (e.g., via cruise ships or the transport of goods) were factored in. Additionally, we assume that early cases in the five destination countries were detected accurately and in a timely fashion. If cases occurred sooner than reported in these receiving countries, the exportation rate from China could have been even higher.
Imai, Natsuko, Ilaria Dorigatti, Anne Cori, Christl Donnelly, Steven Riley, and Neil M. Ferguson, Report 2: Estimating the Potential Total Number of Novel Coronavirus (2019-nCoV) Cases in Wuhan City, China, London: Imperial College London, January 22, 2020.
The COVID-19 Air Traffic Visualization tool combines confirmed COVID-19 case data from the Johns Hopkins University Center for Systems Science and Engineering's COVID-19 Dashboard with detailed air passenger data from the International Air Transport Association's Nationality Traffic Report program. Together, these data sets allow the researchers to visualize and analyze the estimated transmission of the novel coronavirus via air travel, outline the resulting implications, and offer suggestions for minimizing the most-dangerous potential vectors.
This report is part of the RAND Corporation Research report series. RAND reports present research findings and objective analysis that address the challenges facing the public and private sectors. All RAND reports undergo rigorous peer review to ensure high standards for research quality and objectivity.
Our mission to help improve policy and decisionmaking through research and analysis is enabled through our core values of quality and objectivity and our unwavering commitment to the highest level of integrity and ethical behavior. To help ensure our research and analysis are rigorous, objective, and nonpartisan, we subject our research publications to a robust and exacting quality-assurance process; avoid both the appearance and reality of financial and other conflicts of interest through staff training, project screening, and a policy of mandatory disclosure; and pursue transparency in our research engagements through our commitment to the open publication of our research findings and recommendations, disclosure of the source of funding of published research, and policies to ensure intellectual independence. For more information, visit www.rand.org/about/research-integrity.
This document and trademark(s) contained herein are protected by law. This representation of RAND intellectual property is provided for noncommercial use only. Unauthorized posting of this publication online is prohibited; linking directly to this product page is encouraged. Permission is required from RAND to reproduce, or reuse in another form, any of its research documents for commercial purposes. For information on reprint and reuse permissions, please visit www.rand.org/pubs/permissions.
The RAND Corporation is a nonprofit institution that helps improve policy and decisionmaking through research and analysis. RAND's publications do not necessarily reflect the opinions of its research clients and sponsors.
Mouton, Christopher A., Russell Hanson, Adam R. Grissom, and John P. Godges, COVID-19 Air Traffic Visualization: COVID-19 Cases in China Were Likely 37 Times Higher Than Reported in January 2020. Santa Monica, CA: RAND Corporation, 2020. https://www.rand.org/pubs/research_reports/RRA248-3.html.
Mouton, Christopher A., Russell Hanson, Adam R. Grissom, and John P. Godges, COVID-19 Air Traffic Visualization: COVID-19 Cases in China Were Likely 37 Times Higher Than Reported in January 2020, RAND Corporation, RR-A248-3, 2020. As of November 30, 2023: https://www.rand.org/pubs/research_reports/RRA248-3.html