2022 Volume 37 Issue 1 Pages 60-65
Objective: To evaluate COVID-19 spread in the Tokyo area, we monitored monthly seropositivity against SARS-CoV-2 at the outpatient clinic and health screening of the Tokyo Midtown Clinic from June 2020 to October 2021.
Methods: To measure antibodies against SARS-CoV-2, we used Elecsys® anti-SARS-Cov-2 (RUO) system.
Results: From June 2020 to October 2021, 5,315 persons had their first antibody test against SARS-CoV-2. Monthly ratio of seropositivity during the period were 2.0%, 1.2%, 1.2%, 1.5%, 2.9%, 2.7%, 4.1%, 4.3%, 8.8%, 11.3%, 5.9%, 11.3%, 8.3%, 12.6%, 13.7%, 13.8%, respectively. After February 2021, seropositivity was significantly increased. Seropositivities before and after February 15 were 2.5 ± 1.2% and 10.7 ± 2.8% (p=0.001). It reached around 14% when the fifth outbreak started. Analyzing 112 patients who first became seropositive, sixteen (14.3%) were past PCR-positive (A). Fifteen (13.4%) had close contact with COVID-19 patients or cold-like symptoms without positive results of PCR nor diagnostic tests (B). The rest, 81 (72.3%) of them, were asymptomatic (C). Titers of group A were significantly higher than that of group C, 95.3 ± 61.8 vs. 50.7 ± 53.0 (p=0.011). Our seropositivity results were 2.3 times more than the official report in the Tokyo area.
Conclusions: The spread of COVID-19 among socially active adults in Tokyo should be more than expected from PCR positive numbers. They were mostly asymptomatic. There was no case of COVID-19 infection during clinic operations under a strict preventive policy.
