Many variants of SARS-CoV-2 have emerged, and decreased neutralizing antibodies after vaccination and breakthrough infections have become a problem. The importance of monitoring titers of neutralizing antibodies is getting higher. We enrolled 146 COVID-19 patients, who were thought to be infected with Wuhan-hu-1 or D614G strains, and examined the time course of neutralizing titers against six concerning strains (Wuhan-hu-1, Alpha, Beta, Gamma, Kappa, and Delta) using newly developed ELISA. The acquisition of neutralizing titer was positively associated with disease severity. Immune evasions were observed approximately 20 to 30% for Alpha, Kappa, and Delta variant, and 40 to 45% for Beta and Gamma variant. The titers against all strains decreased over time, and interestingly, while titers against Wuhan-hu-1 decreased by 23%, those to Delta variant decreased by 70%. Our simple, cost-effective, and non-hazardous system will be applicable to process numerous samples, such as monitoring titers against prevalent strains after infection or vaccination.
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