Orthogonal antibody testing for COVID-19 among healthcare workers in a non-epidemic place and time: Japan’s Iwate Prefecture, May 18-31, 2020

Of the 47 prefectures in Japan, Iwate had the fewest cases of coronavirus disease 2019 (COVID-19), with the first diagnosis officially confirmed on July 28, 2020. A baseline serological survey of COVID-19 antibodies is essential to accurately evaluate an epidemic outbreak. The primary purpose of this study was to determine pre-epidemic prevalence of COVID-19 antibodies among healthcare workers, using two laboratory-based quantitative tests. In addition, a point-of-care (POC) qualitative test, rapid, simple, and convenient for primary care clinics, was compared with the laboratory-based tests. All antibody tests were performed on serum from 1,000 healthcare workers (mean age, 40 ± 11 years) in Iwate Prefectural Central Hospital, May 29-31, 2020. A COVID-19 case was defined as showing positive results in both laboratory-based quantitative tests. None of 1,000 samples had positive results in both of the laboratory immunoassays. The POC test showed positive results in 33 of 1,000 samples (3.3%) (95% confidence interval: 2.19-4.41), but no samples were simultaneously positive in both laboratory-based tests. In conclusion, COVID-19 cases were not serologically confirmed by a baseline control study of healthcare workers at our hospital in late May, 2020. Moreover, the POC qualitative test may offer no advantage in areas with very low prevalence of COVID-19, due to higher false-positive reactions compared with laboratory-based quantitative immunoassays.