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Recombinant RBD and S-trimer characterization: (A) reducing and non-reducing 10% SDS-PAGE analyses of purified proteins: M—molecular weight marker; 1—S-trimer; 2—RBD; (B) COVID-19-convalescent sera’s reactivity to RBD and S-trimer (mean ± SD). Serum samples of healthy donors were collected before the COVID-19 pandemic, and immune sera were obtained from patients 2–3 weeks after symptom expression and confirmation of the diagnosis by PCR tests for SARS-CoV-2.
