SARS-CoV-2 IgG seroprevalence in the Okinawa Main Island and remote islands in Okinawa, Japan, 2020-2021.

We estimated the seroprevalence of anti-SARS-COV-2 IgG in different island groups in Okinawa. A cross-sectional sero-survey was repeated in three periods between July 2020 and February 2021. A total of 2683 serum samples were collected from six referral medical centers, each covering a separate region in Okinawa. In the main island, the seroprevalence was 0.0% (0/392, 95% CI: 0.0-0.9), 0.6% (8/1448, 0.2-1.1), and 1.4% (8/582, 0.6-2.7) at the 1^st, 2^nd, and 3^rd sero-survey, respectively. In the remote islands, the seroprevalence was 0.0% (0/144, 95% CI: 0.0-2.5) and 1.6% (2/123, 0.2-5.8) at the 2^nd and 3^rd survey, respectively. The case detection ratio was 2.7 (95% CI: 1.3-5.3) in the main island and 2.8 (0.7-11.1) in the remote islands during the 3^rd survey. The case detection ratio was the highest in people aged 20-29 years (8.3, 95% CI: 3.3-21.4) in the main island and in those aged 50-59 years (14.1, 2.1-92.7) in the remote islands, suggesting under-reporting of clinical cases by the surveillance system in these subgroups. A sero-survey during an emerging infectious disease epidemic can be useful for validating the reliability of the surveillance system by providing the case detection ratio.