The effect of inter-city travel restrictions on geographical spread of COVID-19: Evidence from Wuhan, China

Billy J Quilty; Charlie Diamond; Yang Liu; Hamish Gibbs; Timothy W Russell; Christopher I Jarvis; Kiesha Prem; Carl A B Pearson; Samuel J Clifford; Stefan Flasche; Petra Klepac; Rosalind M Eggo; Mark Jit

Many assumptions were made in the formulation of the travel scenarios. The Baidu Huiyan mobility values were presented on a relative index scale; we assumed a linear scaling factor of 50,000 travellers per unit. This was based on widely quoted estimates of people leaving Wuhan and the inter-city capacity of the travel network (7,10-13,21,22) (Supplementary Appendix 1) . However, the index may represent a different number of travellers, or the scale may even be nonlinear and the result of a more complex function, but without other evidence we assume linearity, as have other studies (7, 12) . If we chose a higher scaling factor, similar to ones used in other studies (11, 12) , it is likely that infected travellers would have arrived even earlier and in greater numbers to the four destination cities. Additionally, by reconstructing travel outflows for both dates outside of the observed range (22 Nov -31 Dec) and simulated aspects of our scenarios, i.e. Chunyun affected travel days in non-Chunyun scenarios, the actual travel pattern may not have been accurately represented. Further assumptions were also made surrounding the pairwise travel flows, as observed data was only available for 2020, and the travel flows between Wuhan and each other prefecture-level city may have differed in 2019. We only considered Wuhan to be the sole source of infected individuals, and we only accounted for travellers making single-leg journeys to their destination. As such, we may underestimate the number of infected persons arriving by not considering the number of travellers which may have stopped in an intermediate location, become infected, and then arrived at the destination to seed local transmission, or indeed infected travellers arriving from outside of Wuhan. Hence most of our assumptions likely underestimated the number of travellers from Wuhan, and our conclusions would likely be the same even if the true number was higher. However, we also assumed that individuals would travel regardless of their infection status, which may overestimate the number of infections in destination cities.