Increased SARS-CoV-2 Seroprevalence and the Spread of Cases of Infection Without Awareness Among Healthcare Workers Through 2020-2022 in a Japanese Medical Center

Background: Although more than 80% of Japanese people have been fully vaccinated and more than 60% boosted, the daily number of coronavirus disease 2019 (COVID-19) infections has been high in Japan. Yet studies on the prevalence among Japanese people and the causative factors for rapid spread have remained limited. The aim of this study was to examine the recent point prevalence and associated factors in healthcare workers (HCWs) of a medical center.Methods: This cohort study included HCWs who gave consent for serological analysis during the 2020-2022 annual health check-ups at Juntendo University Hospital in Tokyo. The participants’ vaccination records and SARS-CoV-2 infection history were extracted from the hospital’s electronic records system.Findings: The point prevalence surged from 0∙3% in 2020 and 1∙6% in 2021 to 17∙7% in 2022. Of the 3,788 participating HCWs in year 2022, 669 were seropositive for N-specific antibodies, including 325 (48∙6%; 325/669) without awareness of SARS-CoV-2 infection. Of 357 participants with PCR-confirmed COVID-19 infections in year 2022, 79∙0% (282/357) of the cases were found after January 2022, after the Omicron variant was detected in Tokyo.Interpretation: The rapidly increased prevalence at this medical center indicates the fast spread of the Omicron variant among HCWs. In a highly vaccinated medical center with strict infection control measures, including PCR tests even for close contacts, the high percentage of infection without awareness suggests that lack of awareness may be a key driving factor causing rapid person-to-person transmission.Funding Information: This research was supported by AMED (JP20fk0108472 to TN).Declaration of Interests: Roche Diagnosis provided reagents for the Elecsys Anti-SARS-CoV-2 assay free of cost to YT. All other authors declare no conflict of interests.Ethics Approval Statement This study complied with all the relevant national regulations and institutional policies and was performed in accordance with the tenets of the Declaration of Helsinki. Informed consent was obtained from all study participants. the Institutional Review Board (IRB) of Juntendo University Hospital (IRB #00-000).