Estimating the number of undetected COVID-19 cases among travellers from mainland China.
Bhatia S., Imai N., Cuomo-Dannenburg G., Baguelin M., Boonyasiri A., Cori A., Cucunubá Z., Dorigatti I., FitzJohn R., Fu H., Gaythorpe K., Ghani A., Hamlet A., Hinsley W., Laydon D., Nedjati-Gilani G., Okell L., Riley S., Thompson H., van Elsland S., Volz E., Wang H., Wang Y., Whittaker C., Xi X., Donnelly CA., Ferguson NM.
Background: 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. Methods: 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. Results: 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. Conclusions: Our analysis shows that a large number of COVID-19 cases remain undetected across the world. These undetected cases potentially resulted in multiple chains of human-to-human transmission outside mainland China.