Estimating number of global importations of COVID-19 from Wuhan, risk of transmission outside mainland China and COVID-19 introduction index between countries outside mainland China

Haoyang Sun; Borame Lee Dickens; Mark Chen; Alex Richard Cook; Hannah Eleanor Clapham

Flight data has been used to determine connectivity between countries and therefore risk of onward transmission from China to other countries. Early in the outbreak, Bogoch et al. listed the countries at most risk of importations given the number of flights from Wuhan. 7 Their ranking ultimately followed closely the countries that first reported imported cases, albeit with some countries predicted to report that did not. Since then others have used flight and other data to highlight countries at most risk of importation of SARS-CoV-2 from China. 8, 9 These papers have then used the infectious disease vulnerability index (IDVI) as an assessment of how at risk a country is to local transmission. 8, 9 In countries where importations are occurring, it will be important to quantify the risk of onward transmission occurring and the extent of this transmission. Kucharski et al. 10 considered this generally given the number of importations using a probabilistic model, and Wu et al estimated a probability of transmission within cities outside China. 4 However risk estimates of importations to countries outside China have been focused on the risk of importations from mainland China, with the risk being correlated with the connectivity of places in mainland China to other countries. [7] [8] [9] As transmission increasingly occurs in countries outside mainland China it may also become important to consider the risk of importations to and from other countries. Indeed there have already been cases outside of China, only reporting travel history to countries outside of China, such as from Singapore to the UK, 11 and from Thailand, Singapore and Japan to South Korea. 12 In light of this, there is some urgency to assessing the relative risks of onward transmission between countries outside of China.