*4.2. Recombinant Proteins*

On the basis of the nucleotide sequences of SARS-CoV-2 presented in GenBank: MN908947 [24], the plasmids for the expression of the full N and full S trimer (spike) and also its part in the S1 region, RBD, were constructed. Recombinant proteins of the S trimer and RBD were obtained in eukaryotic cells lines of female hamster ovarian (CHO-K1), and the nucleoproteins were obtained in *E. coli*. All recombinant proteins were purified with affinity chromatography.

#### *4.3. Blood Sera*

In this study, we utilized: 1. the convalescents' blood sera from the employees of the Federal Research Center of the Fundamental and Translational Medicine SB RAS and their relatives who had suffered from mild cases of COVID-19, including asymptomatic cases, as well as moderate COVID-19 cases with pneumonia; 2. blood sera from intensive care unit patients with severe COVID-19, which were retrieved from the Infectious Diseases Clinical Hospital No. 1 in Novosibirsk. The diagnosis of COVID-19 in patients was based on the presence of SARS-CoV-2 RNA, which was detected using the RealBest SARS-CoV-2 RNA kit (Vector-Best, Novosibirsk, Russia) in a certified laboratory for the diagnosis of COVID-19 at the Federal Research Center of the Fundamental and Translational Medicine SB RAS. Permission to use the clinical material was obtained from the Ethics Committee of the Research Center (Protocol No. 17 as of 17 June 2020). The blood sera of convalescents and patients from the Infectious Disease Hospital were tested for the absence of SARS-CoV-2 RNA before conducting this study.
