ELISA and virus neutralization assay
The following experimental steps were applied in the analyses:
  1. The serological analysis of SARS-CoV-2-specific IgG was performed by in-house ELISA. The following SARS-CoV-2 antigens were used: S1 (Acro Biosystems, Newark, DE, USA), RBD (ATUM, Newark, California, USA), S2 (Acro Biosystems, Newark, DE, USA), and N (Acro Biosystems, Newark, DE, USA). Detailed information about the selected antigens is shown in Table S4.
  2. To determine the IBV-specific IgG response, the in-house ELISA procedure was performed using 4/91, IS/1494/06, M41, and D274 strains (Table S5).
  3. For the measurement of IgG specific for SARS-CoV-2 peptides, peptide epitope ELISA were applied (As previously reported by Shrock et al.26). SARS-CoV-2 peptides ORF3a (aa. 172-205), N (aa. 153-176), N (aa. 221-244), N (aa. 358-381), N (aa. 382-405), S (aa. 547-570), S (aa. 782-805), S (aa. 807-830), S (aa. 1138-1161), a rhinovirus A peptide (aa. 567-591), a human herpesvirus 4 peptide (aa. 398-422) and an HIV-1 peptide (aa. 967-991) (Biomatik, Ontario, Canada) were used. Detailed information about the selected peptides is listed in Table S6.
  4. The amino acid sequence of each selected peptide of SARS-CoV-2 and the alignment of corresponding sequences in the endemic HCoVs (NL63, 229E, HKU1, and OC43) and IBV (4/91, M41, and D274), are shown in Figure S8A-H. Sequences were aligned by MUSCLE in JalView (http://www.jalview.org, version 2.11.1.4).
  5. The presence of neutralizing antibodies against SARS-CoV-2 was assessed by live-virus neutralization assay.27
For further details, please refer to the Supplementary Materials and Methods.