Comparison of anti-nucleocapsid antibody assays for the detection of SARS-CoV-2 Omicron vaccine breakthroughs after various intervals since the infection

Antibody assays with the nucleocapsid (NC) protein as the target antigen can identify severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infections when polymerase chain reaction (PCR) analyses are unavailable. Regarding the kinetics of NC-specific antibodies, vaccine breakthroughs with Omicron subvariants may differ from infections with the ancestral wild-type virus. Therefore, we evaluated which assays have the highest sensitivity for detecting NC-specific antibodies after various intervals since breakthrough infections with an Omicron subvariant. The study included 279 samples from vaccinated subjects who experienced PCR-confirmed Omicron breakthrough infections between 21 and 266 days before sampling. The samples were comparatively assessed with the Elecsys (R) Anti-SARS-CoV-2 N (Roche), the Anti-SARS-CoV-2-NCP-ELISA (Euroimmun), the recomLine SARS-CoV-2 IgG (Mikrogen), and the SARS-CoV-2 ViraChip IgG assays (Viramed). In the whole cohort, the Elecsys (R) Anti-SARS-CoV-2 N assay displayed the highest sensitivity (93%, p<0.0001), followed by the recomLine SARS-CoV-2 IgG assay (70%), the SARS-CoV-2 ViraChip IgG assay (41%) and the Anti-SARS-CoV-2-NCP-ELISA (35%). Although measured antibody levels and time-dependent sensitivities differed, the extent of the antibody decrease was similar among all assays. As demonstrated by this study, manufacturer-dependent differences in the sensitivities of NC-specific antibody assays should be considered when serology is applied to link previous SARS-CoV-2 infections with potential post-COVID sequelae.