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

If we assume each country has immediately isolated all the reported cases imported from Wuhan (and therefore truncated transmission), but not isolated the extra cases we estimated and that no extra control measures are put in place, we estimate that the chance that local transmission would occur and sustain for at least three generations exceeds 50% for a total of 16 countries or territories, including four outside Asia: Australia, Canada, United Kingdom, and United States ( Table 2 ). In a second scenario, where we assume that 95% of all the imported cases from Wuhan were immediately isolated, the estimated local outbreak risk reduces substantially, with only Thailand having a local outbreak risk estimate greater than 50%. Still, the estimated risk of local transmission sustaining for at least three generations is not negligible (> 20%) in many other countries or territories, including Japan, Taiwan, Hong Kong, South Korea, United States, Malaysia, and Singapore ( Table 2 for the countries or territories having a local outbreak risk estimate greater than 50% in the main analysis, and Figure 2 & Table S2 for results obtained for all countries or territories outside of mainland China). For countries where we estimate a large number of unreported cases, the local outbreak risk is ranked higher in the first scenario than the second scenario, as the extra estimated cases not being detected means a higher risk of onward transmission compared to countries where we estimate higher detection (Table 2 ).