Geographic correlation between the number of COVID-19 cases and the number of overseas travelers in Japan, Jan-Feb, 2020.

Wagatsuma, Keita; Phyu, Wint Wint; Osada, Hidekazu; W Tang, Julian; Saito, Reiko

Since the coronavirus disease 2019 (COVID-19) emerged in Wuhan, China, in December 2019, it has rapidly spread around the world, the number of patients is also increasing in Japan. The number of COVID-19 cases in Japan in the early stage was not uniform and were largely concentrated in several prefectures. The distribution of the COVID-19 cases and the number of foreign travelers as well as Chinese travelers at prefectural level showed a strong positive correlation coefficient of 0.68 (P<0.0001) and 0.60 (P<0.0001), respectively. Moreover, phylogenetic tree analysis revealed that the all of the registered SARS-CoV-2 detected in from January 23 and through February 29, 2020 belonged to the Chinese lineage, while those detected in March 2020, belonged to American and European lineages. Only 14 (20.3%) were infected outside of Japan, but the majority of the cases (79.7%) were infected domestically. To conclude, higher number of COVID-19 cases were found in prefectures with more Chinese travelers, supporting the importance of enforcing policies to restrict entry of overseas travelers to control COVID-19 spread. These findings highlight the risk of secondary transmission in the community caused by apparent or silent imported cases.