SARS-CoV-2 N protein and anti-spike serologies: insights into COVID-19 disease severity and mortality—a secondary analysis of the ACTIV-1 trial

Background: Understanding factors that predict progression to severe COVID-19 is critical. Antibodies targeting SARS-CoV-2 spike protein confer protection, while the N protein of SARS-CoV-2 plays roles in viral replication and immune dysfunction. This study explores the significance of N protein and anti-spike antibodies on disease severity, progression, and mortality. Objectives: To evaluate the relationship between SARS-CoV-2 N protein and anti-spike antibody levels with disease severity, clinical outcomes, and mortality in hospitalized patients with COVID-19. Design: A secondary analysis of serologic data from participants in the ACTIV-1 randomized clinical trial, which evaluated immunomodulators for the treatment of hospitalized patients with COVID-19. Methods: A subanalysis of the ACTIV-1 immune modulator trial was conducted. Samples collected at randomization were tested for N protein levels and anti-spike antibodies. Logistic regression and linear models were employed to examine the association between serological measures and clinical outcomes, including 28-day mortality as well as progression to high-flow nasal cannula (HFNC) and invasive mechanical ventilation (MV). Results: Among the 496 participants with detectable serum N protein, the median was 1143 ng/dL, and levels decreased from 2559 ng/dL in participants randomized at 6 days of symptom onset to 477.6 ng/dL at 11 days. Higher anti-spike antibody levels were seen as the days from symptom onset progressed or disease severity increased. Greater disease severity at randomization was associated with 28-day mortality, prolonged days of oxygenation, ventilation, hospitalization, and risk of new non-invasive ventilation, HFNC, MV, or extracorporeal membrane oxygenation use. N protein levels were associated with a higher risk of new non-invasive ventilation or HFNC use, longer oxygenation duration, and extended hospitalization. Anti-spike antibody serologies were not associated with clinical outcomes. Conclusion: N protein levels could provide insights into COVID-19 disease progression and prognosis. Further research is needed to explore the clinical implications of these findings to optimize patient care and enhance outcomes.