Seroprevalence of antibodies to SARS‐CoV‐2 in healthcare workers in a nonepidemic region, Japan: A hospital‐based study on May, 2020

Abstract The polymerase chain reaction (PCR) testing rate is low in our local area and the true rate of coronavirus disease 2019 (COVID‐19) infection may include many asymptomatic individuals. We conducted a serosurveillance using antibody testing in an area where official report of COVID‐19 infection is not done yet. Blood samples were obtained from 1404 healthcare workers (41 ± 11 years) in our hospital on May 29–31, 2020. First, the potential infection frequency was confirmed using two quantitative antibody tests. In addition, the usefulness of rapid antibody kit testing for COVID‐19 serosurveillance was examined. A COVID‐19‐indected case was defined as showing positive results in both quantitative tests. None of 1404 samples had positive results from the two quantitative tests. The false positive rates were 0.36% and 0.07%, whereas those in rapid antibody kits were 3.3% and 3.0%. In conclusion, as of May, 2020, potential spread mainly by asymptomatic individuals infected with COVID‐19 was not found in our local area where there was no official report of COVID‐19, even if the PCR testing rate was low. Rapid antibody kits might not be useful due to the high false positive rate in an area with a low incidence of COVID‐19 infected individuals.

were the lowest level in Japan. 5 From the limited PCR testing, the true COVID-19-infected rate may include many asymptomatic individuals.
Iwate Prefectural Central Hospital is in Morioka, the capital city of Iwate and plays a central role in disease treatment and prevention with providing high-level medical research and services for the region's population. An orthogonal testing with two or more quantitative antibody tests with a very high specificity (99.5% or greater) has been useful in populations with a very low prevalence of COVID-19. 6 This study was designed to find out the potential spread mainly by asymptomatic individuals infected with COVID-19 in our local area. For this primary purpose, we conducted an orthogonal quantitative antibody testing with blood samples from healthcare workers in our hospital. In addition, the usefulness of rapid antibody kits for point-of-care (POC) was examined.

| Study population and antibody tests
Blood samples were taken during the annual health checkups of 1706 healthcare workers (physicians, nurses, pharmacists, radiographers, laboratory technicians, and medical office workers) on May 18-29, 2020, and stored at −20°C. Serum samples (n = 1404) from employees with informed consent were analyzed for the detection of antibodies to COVID-19 using the laboratory-based quantitative and POC qualitative tests on May 29-31, 2020.

| DISCUSSION
To the best of our knowledge, this is the first study to evaluate the potential rate of antibodies to COVID-19 in a region having no officially confirmed cases of COVID-19 infection. We obtained blood samples from healthcare workers in our hospital, and quantitatively measured COVID-19 antibodies in serum using two laboratory- Several commercial COVID-19 antibody immunoassays are now available. In a region with very low prevalence, an antibody test with a high specificity (perhaps ≥ 99.5%) should be used, which would yield a higher positive predictive value. 9 This study's laboratorybased quantitative tests had a high specificity (99.6% and 99.8% in Abbott's and Roche's assays, respectively), and positive results were only detected in four (0.36%) samples in the former and one (0.07%) sample in the later. In the rapid antibody kits for POC, there were more false-positive COVID-19 antibody results, whose specificity were relatively low (94.6% and 95.6%). Although the rapid qualitative kits are reliable in areas with a high-prevalence of COVID-19, 10 they have the potential for poor accuracy in low -prevalence area. binding. In this study, attempts to use the positive or negative control test presented difficulties because of no identified infected cases and the stay-at-home order that affected the population. Third, the timing of COVID-19 antibody tests may be significant for accurate COVID-19 antibody detection. Because recent reports showed the disappearance of antibody or a decrease in the titer after COVID-19 infection, 11,12 a repeated measurement of antibody would be warranted after a potent epidemic.

| CONCLUSION
The seroprevalence of the positive COVID-19 antibodies was 0% in a retrospective study with the blood samples from our hospital's healthcare workers in Iwate, where there was no official report of COVID-19. This results shows no potential spread primarily by subclinical infected cases in our area. Laboratory-based quantitative tests with a high specificity, but not rapid antibody kits for POC with a poor specificity, are required for antibody serosurveillance in an area with a low prevalence of COVID-19 cases.

ACKNOWLEDGMENTS
The authors gratefully acknowledge the clinical laboratory technologists in Iwate Prefectural Central Hospital for their technical work with blood sampling and measurement. We would also like to thank Enago (www.enago.jp) for English language editing. This study was supported by no specific grant from any funding agency in the public, commercial, or not-for-profit sectors.