Detection of anti-nucleocapsid antibodies associated with primary SARS-CoV-2 infection in unvaccinated and vaccinated blood donors

Anti-nucleocapsid (N) antibody assays can be used to estimate SARS-CoV-2 infection prevalence in regions employing anti-spike based COVID-19 vaccines. However, poor sensitivity of anti-N assays in detecting infections after vaccination (VI) has been reported. To support serological monitoring of infections, including VI, in a large blood donor cohort (N=142,599), we derived a lower cutoff for identifying previous infection using the Ortho VITROS Anti-SARS-CoV-2 Total-N Antibody assay, improving sensitivity while maintaining specificity >98%. Sensitivity was validated in samples donated after self-reported infections diagnosed by a swab-based test. Sensitivity for first infections in unvaccinated donors was 98.1% (95% CI: 98.0,98.2) and for VI was 95.6% (95.6,95.7), using the standard cutoff. Regression analysis showed sensitivity was reduced in the Delta compared to Omicron period, in older donors, asymptomatic infections, ≤30 days after infection and for VI. The standard Ortho anti-N threshold demonstrated good sensitivity, which was modestly improved with the revised cutoff.