Background: The third wave of the COVID-19 epidemic in the island of Hokkaido, the second largest island in Japan, began abruptly in October 2020. Methods: We conducted a phylodynamic analysis of the SARS-CoV-2 genome sequences obtained from tertiary medical centers in the Greater Tokyo Area and Sapporo, the largest city in the island of Hokkaido, and genome sequences published by GISAID, an international SARS-CoV-2 genome database. We also analyzed the statistics on the person-nights of travelers in the island of Hokkaido from the Greater Tokyo Area in 2019 versus 2020. Results: At least eight sub-lineages belonging to the B.1.1.214 lineage were introduced to the island of Hokkaido from the island of Honshu, the mainland of Japan from late July to November 2020, during the governmental travel promotion program. Five of the eight sub-lineages originated from the Greater Tokyo Area. Comparison of the monthly ratios of the person-nights of travelers in the island of Hokkaido from the Greater Tokyo Area in 2019 and 2020 revealed that the highest value occurred in October 2020. Conclusion: We contend that the Japanese governmental travel promotion program contributed to the introduction of the B.1.1.214 sub-lineages from the main island of Honshu to the island of Hokkaido, and drove the third wave in Hokkaido, even if we are unable to establish the causality.
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