Epidemiology of coronavirus disease 2019 in Yamagata Prefecture, Japan, January-May 2020: The importance of retrospective contact tracing.

Seto, Junji; Aoki, Yoko; Komabayashi, Kenichi; Ikeda, Yoko; Sampei, Mika; Ogawa, Naomi; Uchiumi, Yumiko; Fujii, Shunji; Chiba, Masami; Suzuki, Emiko; Takahashi, Tatsuya; Yamada, Keiko; Otani, Yoshiko; Ashino, Yoshihiro; Araki, Kyoko; Kato, Takeo; Ishikawa, Hitoshi; Ikeda, Tatsuya; Abe, Hideaki; Ahiko, Tadayuki; Mizuta, Katsumi

Public health interventions have served an important role in controlling coronavirus disease 2019 (COVID-19), a rapidly spreading infectious disease. To contribute to future COVID-19 countermeasures, we aimed at verifying the results of countermeasures achieved by public health centers (PHCs) against the first wave of COVID-19 in Yamagata Prefecture, Japan. During January-May 2020, 1,253 patients suspected of SARS-CoV-2 infection were invited for testing. Simultaneously, based on retrospective contact tracings, PHCs investigated the infection sources and transmission routes of laboratory-confirmed COVID-19 cases and tested 928 contacts. Consequently, 69 cases were confirmed during March 31 - May 4, 58 (84.1%; 95% confidence interval 75.5-92.7) of whom were found from contacts. The spread of infection was triggered by cases harboring epidemiological links outside of Yamagata. Then, the number of cases increased rapidly. However, PHCs identified epidemiological links in 61 (88.4%; 95% confidence interval 80.8-96.0) of the 69 cases and transmission chains up to the fifth generation. Finally, the spread of infection ended after approximately one month. Our results indicate that the identification of infection sources and active case finding from contacts based on retrospective contact tracing was likely to be an effective strategy to end the first wave of COVID-19 in Yamagata.