Seroprevalence of Antibodies to SARS-CoV-2 in Healthcare Workers in Non-epidemic Region: A Report from Iwate Prefecture in Japan

Background: As of June 18, 2020, Iwate is the only one of 47 prefectures in Japan with no confirmed coronavirus disease 2019 (COVID-19) cases. Serological survey for COVID-19 antibodies is crucial in area with low prevalence as well as epidemic area when addressing health and social issues caused by COVID-19. Rapid, accurate and easy-to-use antibody tests as well laboratory-based antibody tests are necessary for confirming immunity in a given community. Methods:Serum samples from healthcare workers (n = 1,000, mean 40 ± 11 years) of Iwate Prefectural Central Hospital, Iwate, Japan were tested for the prevalence of the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) antibodies. Two laboratory-based quantitative tests (Abbott Architect SARS-CoV-2 IgG and Roche Elecsys Anti-SARS-CoV-2 assays) and one point-of-care (POC) qualitative test (Alfa Instant-view plus COVID-19 Test) were performed simultaneously. Sensitivity and specificity were 100%, 99.6% in Abbott assay; 100%, 99.8% in Roche assay; 97.8%, 94.6% in Alfa POC test, respectively. Results:The laboratory-based quantitative tests showed positive in 4 of 1,000 samples (0.4%) (95% CI: 0.01 to 0.79): 4/1,000 (0.4%) (95% CI: 0.01 to 0.79) in Abbott; 0/1,000 (0%) in Roche. Positive samples were not detected for both Abbott and Roche assays. The POC qualitative test showed positive in 33 of 1,000 samples (3.3%) (95% CI: 2.19 to 4.41), showing higher rates than those of the laboratory-based quantitative tests. There were no samples with simultaneous positive reaction for two quantitative tests and a POC test. Conclusions: Infected COVID-2 cases were not confirmed by a retrospective serological study in healthcare workers of our hospital. The POC qualitative tests with lower specificity have the potential for higher false positive reactions than the laboratory-based quantitative tests in areas with very low prevalence of COVID-19.