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

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.