What Can We Learn from the Exported Cases in Detecting Disease Outbreaks: A Case Study of the COVID-19 Epidemic

Background: Travel is a potent force in the emergence of disease. Early warning in the travel origins is crucial to prevent the disease from spreading. However, many travel origins have delays in reporting the disease outbreak. Under this circumstance, those exported cases (travelers tested positive outside their travel origins) became valuable data for detecting the disease outbreaks. Methods: We estimated some key indicators (the exponential growth rate, the doubling time, and the basic reproduction number) using the exported cases in the early phase of the COVID-19 epidemic of four travel origins, Wuhan (China), U.S., Italy, and Iran. We compared the estimates from the exported cases to the ones from domestic data, and quantitatively evaluated how different features in the exported cases influenced the estimation. We used hypothesis testing to determine if an indicator has exceeded a certain threshold.Findings: We found that the dates that the exported cases rang an alarm about the disease outbreak were all before or shortly after some emergency measures were taken in the travel origins. If the assumed epidemic start date was relatively accurate and the traveler samples were representative of the general population, the indicators estimated from the exported cases were consistent with the domestic data. We provided the minimum number of exported cases to determine whether the epidemic growth rate was above a certain level. Flexible scenarios were provided in a Shiny App. Interpretation: Rapid detection of a disease outbreak is crucial for government intervention and raising public awareness. Utilizing the diagnosis resources from all countries, the exported case data is a good addition to the domestic data. To strengthen the global infectious disease surveillance system, we advocate that countries work in a collaborative way by sharing detailed travel data in a timely manner.Funding: NIH/NIAID 5R01AI136664Declaration of Interests: The authors declare no competing interests.