**Appendix A**

**Figure A1.** Specific interaction of IgG convalescents with inactivated viral preparations and recombinant proteins in ELISA. Note. Blood sera: No. 1—an asymptomatic case; No. 2–14—mild cases, No. 15—moderate case with unilateral pneumonia; No. 16—moderate case with hospitalization for bilateral pneumonia; No.17–32—severe cases of patients who were treated in the intensive care unit. Data on the neutralizing activity of these blood sera are presented in Tables S1 and S2.

**Figure A2.** Immunoblotting of viral proteins of the SARS-CoV-2/human/RUS/Nsk-FRCFTM-1/2020 (2020) and SARS-CoV (2002) strains with antibodies of convalescents' blood sera (asymptomatic, mild or moderate cases of COVID-19, as well as patients in the intensive care unit). (**A**). The SARS-CoV-2/human/RUS/Nsk-FRCFTM-1/2020 antigen on the membrane. (**B**). The SARS-CoV antigen (2002) on the membrane. Blood sera: No. 1—an asymptomatic case; No. 2–14—mild cases, No. 15—a moderate case with unilateral pneumonia; No. 16—a moderate case with hospitalization for bilateral pneumonia; No. 17–32—severe cases of patients in the intensive care unit. K- (negative control)—blood serum of a healthy person. The numbers of paired blood sera are underlined.

**Figure A3.** Titers of specific IgG interactions in patients with severe cases of COVID-19 who subsequently died in hospital, with inactivated viral preparations and recombinant proteins in ELISA. Note: data on blood sera numbers are presented in Tables S1 and S3.

**Figure A4.** Immunoblotting of viral proteins of the SARS-CoV-2/human/RUS/Nsk-FRCFTM-1/2020 (2020) and SARS-CoV (2002) strains with antibodies of blood sera of patients who were treated in the intensive care unit diagnosed with COVID-19 (later died in the hospital). (**A**) SARS-CoV-2/human/RUS/Nsk-FRCFTM-1/2020 antigen on the membrane. (**B**) SARS-CoV antigen (2002) on the membrane. *K-* (negative control)—blood serum of a healthy person. The numbers of paired blood sera are underlined.

#### **References**

