Impaired systemic nucleocapsid antigen clearance in severe COVID-19

Objectives: Circulating nucleocapsid (NCP) antigen of SARS-CoV-2 is increased in severely ill COVID-19 patients. However, clinical deterioration of COVID-19 often happens about one week after benign initial presentation. The role of NCP antigenemia as a biomarker in those cases remains unclear. We investigated NCP clearance kinetics in hospitalized patients as a risk assessment tool for predicting necessity of intensive care treatment of COVID-19 patients. Methods: Serum NCP was quantified using a commercial NCP-specific ELISA in hospitalized COVID-19 patients (n=63) during their hospital stay. Results were correlated to COVID-19 disease severity, inflammation parameters, antibody response and results of SARS-CoV-2 PCR from nasopharyngeal swabs. Results: We demonstrate that NCP antigen levels in serum remained elevated in 45.6% of patients requiring treatment on intensive care units (ICU) after >8 days post positive SARS-CoV-2 PCR, compared to complete clearance in all non-ICU patients. This was in contrast to mucosal clearance of virus as measured by PCR. Antigen clearance was associated with higher IgG against S1 but not NCP. Conclusions: Detection of NCP antigenemia after 8 days post COVID-19 diagnosis identifies patients who will require intensive care. Lack of NCP clearance after one week can thus help to assess the risk to develop severe COVID-19.