Serologic Survey of IgG Against SARS-CoV-2 Among Hospital Visitors Without a History of SARS-CoV-2 Infection in Tokyo, 2020–2021
Takahiro Sanada, Tomoko Honda, Fumihiko Yasui, Kenzaburo Yamaji, Tsubasa Munakata, Naoki Yamamoto, Makoto Kurano, Yusuke Matsumoto, Risa Kohno, Sakiko Toyama, Yoshiro Kishi, Takuro Horibe, Yudai Kaneko, Mayumi Kakegawa, Kazushige Fukui, Takeshi Kawamura, Wang Daming, Chungen Qian, Fuzhen Xia, Fan He, Syudo Yamasaki, Atsushi Nishida, Takayuki Harada, Masahiko Higa, Yuko Tokunaga, Asako Takagi, Masanari Itokawa, Tatsuhiko Kodama, Michinori Kohara
Abstract
BACKGROUND: Tokyo, the capital of Japan, is a densely populated city of >13 million people, so the population is at high risk of epidemic severe acute respiratory coronavirus 2 (SARS-CoV-2) infection. A serologic survey of anti-SARS-CoV-2 IgG would provide valuable data for assessing the city's SARS-CoV-2 infection status. Therefore, this cross-sectional study estimated the anti-SARS-CoV-2 IgG seroprevalence in Tokyo. METHODS: Leftover serum of 23,234 hospital visitors was tested for antibodies against SARS-CoV-2 using an iFlash 3000 chemiluminescence immunoassay analyzer (Shenzhen YHLO Biotech, Shenzhen, China) with an iFlash-SARS-CoV-2 IgG kit (YHLO) and iFlash-SARS-CoV-2 IgG-S1 kit (YHLO). Serum samples with a positive result (≥10 AU/mL) in either of these assays were considered seropositive for anti-SARS-CoV-2 IgG. Participants were randomly selected from patients visiting 14 Tokyo hospitals between September 1, 2020 and March 31, 2021. No participants were diagnosed with coronavirus disease 2019 (COVID-19), and none exhibited COVID-19-related symptoms at the time of blood collection. RESULTS: The overall anti-SARS-CoV-2 IgG seroprevalence among all participants was 1.83% (95% confidence interval [CI], 1.66-2.01%). The seroprevalence in March 2021, the most recent month of this study, was 2.70% (95% CI, 2.16-3.34%). After adjusting for population age, sex, and region, the estimated seroprevalence in Tokyo was 3.40%, indicating that 470,778 individuals had a history of SARS-CoV-2 infection. CONCLUSIONS: The estimated number of individuals in Tokyo with a history of SARS-CoV-2 infection was 3.9-fold higher than the number of confirmed cases. Our study enhances understanding of the SARS-CoV-2 epidemic in Tokyo.