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

ABSTRACT

We estimated the seroprevalence of anti-SARS-COV-2 IgG in different island groups in Okinawa and described its changes over time. A cross-sectional sero-survey was repeated in three distinct 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. Patients who visited the emergency department for any reason and underwent blood collection were eligible for the study. Samples were analyzed using an FDA-authorized two-step enzyme-linked immunosorbent assay (ELISA) protocol. The case detection ratio was computed by dividing the seroprevalence by the attack rate obtained from publicly available surveillance data. 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 1st, 2nd, and 3rd 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 2nd and 3rd survey, respectively. The overall case detection ratios at the 3rd survey were 2.7 (95% CI 1.3-5.3) in the main island and 2.8 (0.7-11.1) in the remote islands. The highest age-specific case detection ratio was observed 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. The low seroprevalence at the latest survey suggested that a large-scale epidemic had not yet occurred in Okinawa by February 2021. The case detection ratios imply that the cumulative number of incident cases in Okinawa should be 2-3 times higher than that reported by routine surveillance. The ratio was particularly high in young people probably due to a frequent asymptomatic/mild COVID-19 disease in this age group. To accurately measure the scale of the COVID-19 epidemic, it is crucially important to conduct a sero-survey targeting the young.