<ns4:p><ns4:bold>Background:</ns4:bold> As of August 2021, every region of the world has been affected by the COVID-19 pandemic, with more than 196,000,000 cases worldwide.</ns4:p><ns4:p> <ns4:bold>Methods: </ns4:bold>We analysed COVID-19 cases among travellers from mainland China to different regions and countries, comparing the region- and country-specific rates of detected and confirmed cases per flight volume to estimate the relative sensitivity of surveillance in different regions and countries.</ns4:p><ns4:p> <ns4:bold>Results: </ns4:bold>Although travel restrictions from Wuhan City and other cities across China may have reduced the absolute number of travellers to and from China, we estimated that up to 70% (95% CI: 54% - 80%) of imported cases could remain undetected relative to the sensitivity of surveillance in Singapore. The percentage of undetected imported cases rises to 75% (95% CI 66% - 82%) when comparing to the surveillance sensitivity in multiple countries.</ns4:p><ns4:p> <ns4:bold>Conclusions: </ns4:bold>Our analysis shows that a large number of COVID-19 cases remain undetected across the world.<ns4:bold> </ns4:bold>These undetected cases potentially resulted in multiple chains of human-to-human transmission outside mainland China.</ns4:p>