Measurement of neutralization antibody levels is useful to predict protective immunity in patients who have recovered from COVID-19 and in those who have received vaccines ( Khoury et al., 2021). Waning of acquired immunity is a concern, and whether to administer a booster vaccine is another question that requires addressing ( Baraniuk, 2021). While novel vaccines for SARS-CoV-2 have helped control the pandemic, there are new variants capable of escaping immunogenicity acquired by natural infection and/or vaccination ( Abdool Karim and de Oliveira, 2021 Gupta, 2021 Hacisuleyman et al., 2021). Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), causing coronavirus disease-19 (COVID-19), has caused more than 5 million deaths globally as of November 2021 ( WHO, 2021). In conclusion, semi-quantitative immunoassay kits targeting the RBD showed neutralizing activity that was correlated by titer measurement of anti-NP antibodies would be useful for determining past infections. Due to the slowly waning titer of anti-NP antibodies, the correlation with PRNT titer at the convalescent phase was poor. It also showed the best linear correlation with PRNT titer in both the acute and convalescent phases (Pearson’s R 0.81 and 0.72, respectively). Quantitative titer correlation for high PRNT titers of ND 50 ≥ 50, 200, and 1,000 was investigated with new cut-off values the anti-RBD IgG antibody kit showed the best performance. The representativeness of the neutralizing activity of PRNT ND 50 ≥ 20 was comparable among the four immunoassays (Cohen’s kappa ≈ 0.80). All three chemiluminescent assays showed high analytical performance for the detection of SARS-CoV-2 infection, with a sensitivity ≥ 98% and specificity ≥ 99% those of the sVNT were slightly lower. We measured the total antibodies for the receptor-binding domain (RBD) of the spike protein, total antibodies for the nucleocapsid protein (NP), and IgG antibodies for the RBD. We evaluated the analytical performance and PRNT titer-correlation of one surrogate virus neutralization test (sVNT) kit and three chemiluminescent assays. 9Division of Infectious Diseases, Department of Medicine, Kangbuk Samsung Hospital, Sungkyunkwan University School of Medicine, Seoul, South Koreaįor the clinical application of semi-quantitative anti-SARS-CoV-2 antibody tests, the analytical performance and titer correlation of the plaque reduction neutralization test (PRNT) need to be investigated.8Division of Infectious Diseases and Immunodeficiency, Department of Pediatrics, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, South Korea.7Department of Critical Care Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, South Korea.6Division of Emerging Virus and Vector Research, National Institute of Health, Korea Disease Control and Prevention Agency, Cheongju, South Korea.5Asia Pacific Foundation for Infectious Diseases (APFID), Seoul, South Korea.4Department of Laboratory Medicine, Konkuk University Medical Center, Konkuk University School of Medicine, Seoul, South Korea.3Division of Infectious Diseases, Department of Medicine, Konkuk University Medical Center, Konkuk University School of Medicine, Seoul, South Korea.2Division of Infectious Diseases, Department of Medicine, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, South Korea.1Department of Laboratory Medicine and Genetics, Samsung Medical Center, Sungkyunkwan University School of Medicine, Seoul, South Korea.Beomki Lee 1†, Jae-Hoon Ko 2†, Jiho Park 3†, Hee-Won Moon 4, Jin Yang Baek 5, Sunhee Jung 6, Hee-Young Lim 6, Kyung-Chang Kim 6, Kyungmin Huh 2, Sun Young Cho 2, Cheol-In Kang 2, Doo Ryeon Chung 2, Hee Jae Huh 1, Chi Ryang Chung 7, Yae-Jean Kim 8, Eun-Jeong Joo 9*‡, Eun-Suk Kang 1*‡ and Kyong Ran Peck 2*‡
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