Automated Western immunoblotting detection of anti-SARS-CoV-2 serum antibodies.

Edouard, S.; Jaafar, R.; Orain, N.; Parola, P.; Colson, P.; La Scola, B.; Fournier, P.-E.; Raoult, D.; Drancourt, M.

ELISA and chemiluminescence serological assays for COVID-19 are currently incorporating only one or two SARS-CoV-2 antigens. We developed an automated Western immunoblotting as a complementary serologic assay for COVID-19. The Jess Simple Western system, an automated capillary-based assay was used, incorporating an inactivated SARS-CoV-2 lineage 20a strain as antigen, and IgT detection. In total, 602 sera were tested including 223 from RT-PCR-confirmed COVID-19 patients, 76 from patients diagnosed with seasonal HCoVs and 303 from coronavirus-negative control sera. We also compared this assay with the EUROIMMUN(R) SARS-CoV-2 IgG ELISA kit. Among 223 sera obtained from RT-PCR-confirmed COVID-19 patients, 180/223 (81%) exhibited reactivity against the nucleocapsid and 70/223 (31%) against the spike protein. Nucleocapsid reactivity was further detected in 9/76 (14%) samples collected from patients diagnosed with seasonal HCoVs and in 15/303 (5%) coronavirus-negative control samples. In the subset of sera collected more than 2 weeks after the onset of symptoms, the sensitivity was 94% and the specificity 93%, the latter value probably reflecting cross-reactivity of SARS-CoV-2 with other coronaviruses. The automated Western immunoblotting presented a substantial agreement (90%) with the compared ELISA (Cohen's Kappa=0.64). Automated Western immunoblotting may be used as a second line test to monitor exposition of people to HCoVs including SARS-CoV-2.