Post-hoc evaluation of travel restrictions during the COVID-19 pandemic in Japan, 2020-2022.

BACKGROUND: From February 2020, Japan began implementing travel restrictions against COVID-19. Because such restrictions inevitably have a socioeconomic impact, it is vital to evaluate the impact of the reduced international travel volume. We retrospectively evaluated travel restrictions implemented against COVID-19 in Japan during 2020-2022 by analyzing entry screening results and building a counterfactual model. METHODS: By analyzing weekly numbers of tests and positive test results (October 2020-October 2022) and the total number of border entries by month (2017-2019), we performed a causal inference experiment using an epidemiological model comprising two steps: (i) estimation of risk of infection among travelers and (ii) scenario modeling of a counterfactual volume of travelers using border entry statistics. Under the assumption that the infection risk among travelers was identical between observed and counterfactual scenarios, we estimated absolute difference and relative reduction in imported infections based on statistical models. RESULTS: The estimated counterfactual number of border entries in the absence of pandemic-related changes in travel volume was approximately 49.5 million from October 2020-May 2022, whereas approximately 1.0 million border entries were observed, corresponding to an estimated reduction of 48.5 million entries (approximately 98%) relative to the counterfactual scenario. Under the assumption that the infection risk among travelers was identical between the observed and counterfactual scenarios, this reduction in travel volume corresponded to 530,000-620,000 fewer imported infections. The largest estimated difference in importations to Japan was associated with travel from the Western Pacific region. CONCLUSIONS: The reduced international travel volume was associated with an estimated 530,000-620,000 fewer imported infections in Japan than under the counterfactual scenario.