Increased SARS-CoV-2 seroprevalence and spread of infection without awareness among healthcare workers through 2020–2022 in a Japanese medical center

Abstract

Despite Japan’s high vaccination coverage, daily numbers of new COVID-19 cases have been high. However, studies on the seroprevalence among Japanese people and the causative factors for rapid spread have remained limited. In this study, we aimed to examine the seroprevalence and associated factors in healthcare workers (HCWs) of a medical center in Tokyo using blood samples drawn at annual check-ups from 2020 to 2022. We found that of the 3,788 HCWs in 2022 (by mid-June), 669 were seropositive for N-specific antibodies (tested by Roche Elecsys Anti-SARS-CoV-2 assay); the seroprevalence surged from 0.3% in 2020 and 1.6% in 2021 to 17.7% in 2022. Notably, our study found 325 (48.6%; 325/669) cases were infected without awareness. Among those with a previously PCR-confirmed SARS-CoV-2 infection during the past three years, 79.0% (282/357) were found after January 2022, after the Omicron variant was first detected in Tokyo at the end of 2021. This study indicates the fast spread of the SARS-CoV-2 among HCWs during the Omicron surge in Japan. The high percentage of infection without awareness may be a key driving factor causing rapid person-to-person transmission, as shown in this medical center with high vaccination coverage and strict infection control measures.

Seroprevalence of SARS-CoV-2 antibodies among Japanese healthcare workers from 2020 to 2022 as assayed by two commercial kits

“…In the other words, 98.4% of our HCWs were infection-naïve by mid-2021 19 – 22 . We also reported that by mid-June 2022, the seroprevalence rate increased to 16.7%, mainly due to spread of the Omicron variant 23 . In these studies, data was obtained from annual occupational health physical samples and all were tested using a total antibody assay (Roche Elecsys Anti-SARS-CoV-2).…”

“…The total antibody assay was used to test sera at each year’s health physicals from 2020 through 2022 for all participating healthcare workers. N-positivity results identified in this total antibody assay for those 3 years have been published 19 , 20 , 23 . In this study, the IgG-only assay was used to test all 2538 samples at the 2022 health physical.…”

A comparison of the clinical performance of two commercially available SARS- CoV-2 antibody assays among Japanese healthcare workers (2020–2022)

“…By mid-November 2022, Japan's infection rate reached 19.0% (23.3 million cumulative cases out of 122.8 million in the national population, including those with multiple infections) [5]. In contrast, a recent national N-antibody seroprevalence study in Japan (n = 8,260) of donated blood revealed a national infection rate of approximately 26.5% by mid-November 2022 [6].…”

“…We recently reported that 16.7% of healthcare workers in Juntendo University Hospital of Japan, where P zer/BioNTech BNT162b2 and Moderna mRNA-1273 were used for vaccination, were N-speci c seropositive by mid-June 2022 (n = 3,788) [19]. In that study, data was obtained from annual occupational health physical samples and all were tested using a total antibody assay (Roche Elecsys ® Anti-SARS-CoV-2).…”