by Zhenxiao Ren, Mitsuhiro Nishimura, Lidya Handayani Tjan, Koichi Furukawa, Yukiya Kurahashi, Silvia Sutandhio, Kaito Aoki, Natsumi Hasegawa, Jun Arii, Kenichi Uto, Keiji Matsui, Itsuko Sato, Jun Saegusa, Nonoka Godai, Kohei Takeshita, Masaki Yamamoto, Tatsuya Nagashima, Yasuko Mori
Continuous appearance of SARS-CoV-2 variants and mass vaccination have been intricately influencing on the COVID-19 situation. To elucidate the current status in Japan, we analyzed totally 2,000 sera in August (n = 1,000) and December (n = 1,000) 2021 collected from individuals who underwent a health check-up. The anti-N seropositive rate were 2.1% and 3.9% in August and December 2021, respectively, demonstrating a Delta variant endemic during that time; it was approximately twofold higher than the rate based on the PCR-based diagnosis. The anti-S seropositive rate was 38.7% in August and it reached 90.8% in December, in concordance with the vaccination rate in Japan. In the December cohort, 78.7% of the sera showed neutralizing activity against the Delta variant, whereas that against the Omicron was much lower at 36.6%. These analyses revealed that effective immunity against the Delta variant was established in December 2021, however, prompt three-dose vaccination is needed to overcome Omicron’s outbreak.