Enhanced case finding and self-isolation measures in the early phase of SARS-CoV-2 Omicron transmission, Osaka, Japan, December 2021–January 2022

Miho Kobayashi; Kensaku Kakimoto; Yuichiro Yahata; Yusuke Kobayashi; Hitomi Nagai; Chisato Tanikake; Kazumi Fukumura; Keiko Date; Hiromi Murata; Sae Kitagawa; Yuki Yoshida; Yui Kamoda; Miho Akazaki; Masaaki Tanabe; Chika Shirai; Tomoe Shimada; Taro Kamigaki; Tsuyoshi Sekizuka; Makoto Kuroda; Tomimasa Sunagawa

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Enhanced case finding and self-isolation measures in the early phase of SARS-CoV-2 Omicron transmission, Osaka, Japan, December 2021–January 2022

Author: Miho Kobayashi ^{1
,

2} ; Kensaku Kakimoto ³ ; Yuichiro Yahata ⁴ ; Yusuke Kobayashi ^{4
,

5} ; Hitomi Nagai ⁶ ; Chisato Tanikake ⁷ ; Kazumi Fukumura ⁷ ; Keiko Date ⁷ ; Hiromi Murata ⁷ ; Sae Kitagawa ⁷ ; Yuki Yoshida ⁷ ; Yui Kamoda ⁷ ; Miho Akazaki ⁷ ; Masaaki Tanabe ⁸ ; Chika Shirai ⁹ ; Tomoe Shimada ⁴ ; Taro Kamigaki ¹⁰ ; Tsuyoshi Sekizuka ¹¹ ; Makoto Kuroda ¹¹ ; Tomimasa Sunagawa ⁴
Author Information

1. Field Epidemiology Training Program, National Institute of Infectious Diseases, Tokyo, Japan
2. Gunma Prefectural Institute of Public Health and Environmental Sciences, Gunma, Japan
3. Osaka Institute of Public Health, Osaka, Japan
4. Center for Field Epidemic Intelligence, Research and Professional Development, National Institute of Infectious Diseases, Tokyo, Japan
5. Center for Surveillance, Immunization and Epidemiologic Research, National Institute of Infectious Diseases, Tokyo, Japan
6. Ibaraki Public Health Center, Osaka, Japan
7. Moriguchi Public Health Center, Osaka, Japan
8. Neyagawa City Public Health Center, Osaka, Japan
9. Hirakata City Public Health Center, Osaka, Japan
10. Center for Surveillance, Immunization and Epidemiologic Research, National Institute of Infectious Diseases, Tokyo, Japan
11. Center for Pathogen Genomics, National Institute of Infectious Diseases, Tokyo, Japan
Publication Type:Journal Article
Keywords: COVID-19; SARS-CoV-2; disease outbreaks; contact tracing; epidemiology
From: Western Pacific Surveillance and Response 2025;16(2):29-38
CountryWHO-WPRO
Language:English
Abstract: Objective: The severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant B.1.1.529 (Omicron) was first detected in Japan in November 2021. In Osaka, public health centres subsequently increased active case finding and encouraged self-isolation. This study investigated the effectiveness of these countermeasures. Methods: Cases targeted for analysis were persons who had neither recently travelled abroad nor had contact with foreign tourists but tested positive for SARS-CoV-2 between 24 November 2021 and 4 January 2022 and were suspected or confirmed to have the Omicron variant. We performed a descriptive analysis and calculated the reproduction number (R) for each generation using the branching process method. Genomic sequencing data were analysed to plot a haplotype network. Results: A total of 251 cases were analysed. The median age was 30 years, and 46% (115/251) were in their 20s or younger. The first Omicron case in Osaka was detected on 21 December 2021. Local public health centres conducted health monitoring and contact tracing. We analysed R, using information from six clusters, including 42 pairs with a clear relationship between the case and the infected contact (infector–infectee pairs); the clusters had 19, 21 and 2 cases in each subsequent generation. The basic R (t = 0) was estimated to be 3.2, and subsequent generations (t = 1, 2) of R decreased to 1.1 and 0.1, respectively. The haplotype network showed that these cases constituted a monophyletic group with others detected around Osaka, indicating that these case-related clusters had been contained and were not involved in the nationwide Omicron waves. Discussion: Active case finding and self-isolation were found to be effective in limiting the spread of an emerging novel variant.
Full text:2025082910024998170wpsar-16-1139 Kobayashi FINAL.pdf