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SARS-CoV-2 nucleocapsid antigen levels and patient outcomes

In a recent study published in the Annals of Internal Medicine, researchers assessed the correlation between severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) nucleocapsid (N) antigen levels and hospitalized patient outcomes.

Study: The Association of Baseline Plasma SARS-CoV-2 Nucleocapsid Antigen Level and Outcomes in Patients Hospitalized With COVID-19. Image Credit: PHOTOCREO Michal Bednarek/Shutterstock

Studies have highlighted the higher levels of plasma SARS-CoV-2 ribonucleic acid (RNA) noted during hospitalization due to coronavirus disease 2019 (COVID-19). These levels are associated with a higher prevalence of critical illness, non-pulmonary organ failure, as well as intensive care unit (ICU) admission and hospital mortality. While risk factors like older age, male gender, and diabetes have been established for poor outcomes for SARS-CoV-2 infections, extensive research is needed to estimate the relationship between these factors and plasma viral burden. 

About the study

In the present study, researchers determined whether the levels of plasma SARS-CoV-2 N antigen could estimate short-term patient outcomes.

The study cohort included patients randomly assigned to be treated with either bamlanivimab, sotrovimab, amubarvimab-romlusevimab, tixagevimab-cilgavimab, or Molecular Partners MP0420, or a matched placebo. Quantitative plasma SARS-CoV-2 N antigen was estimated via a microbead-based immunoassay, while anti-spike neutralizing antibodies were assessed with a surrogate viral neutralization test. This assay facilitated the calculation of the percentage of binding blocked by the antibodies in the patient, with a result of 30% binding or above being considered positive. Furthermore, anti-N pan-immunoglobulin positivity was assessed.

The team also extracted SARS-CoV-2 viral RNA from a mid-turbinate nasal swab which was obtained along with the plasma sample. The presence of the SARS-CoV-2 Delta variant in the sample was determined via a reverse transcription–polymerase chain reaction (RT-PCR) assay designed for the N-terminal domain region of the SARS-CoV-2 spike gene.

The eligible patients were hospitalized due to acute COVID-19 within 12 days of onset of COVID-19 symptoms. The patient cohort was categorized according to: (1) enrollment dates; (2) demographic characteristics such as age, gender, ethnicity, or race; (3) geographic region of recruitment such as the US, Asia, Europe, or Africa; (4) coexisting comorbidities such as diabetes, hypertension, renal impairment, chronic obstructive pulmonary disease and/or asthma, cardiovascular disease, obesity, cancer, and immune dysfunction-associated disorders; (5) prior SARS-CoV-2 vaccination history; (6) duration since symptom onset; (7) pulmonary disease severity; and (8) treatment with remdesivir before enrollment.      


A total of 2694 patients were enrolled in the study from August 2020 to November 2021. Among these, 52% of the baseline samples tested positive for the SARS-CoV-2 anti-spike antibody at the time of study enrollment, while 62% were positive for the anti-N antibody. On the other hand, 28% tested negative for both anti-N and anti-spike antibodies. Participants who were enrolled from January 2021 onwards included 88% who tested positive for SARS-CoV-2 Delta variant infection.

The study showed that plasma antigen levels were significantly associated with pulmonary disease severity. Furthermore, the plasma antigen levels were 3.10 times higher among patients that required non-invasive ventilation (NIV) or high-flow nasal cannula (HFNC), 2.88 times higher in those who needed 4L or more conventional oxygen supplementation, and 1.77 times higher in those who needed less than 4L conventional oxygen supplementation in comparison to those who breathed room air.

The plasma antigen levels were higher among patients who had over one week of symptoms which decreased with increasing duration since hospitalization and two or more days of remdesivir treatment. Furthermore, older age was considerably associated with higher plasma antigen levels, especially among patients aged over 65 years. Plasma antigen levels were also higher among those suffering from renal impairment. Notably, Delta-infected patients displayed higher plasma antigen levels than non-Delta patients.

Furthermore, the team noted that 57% of the patients had increased plasma antigen levels. For patients who received a placebo, the chances of pulmonary illness worsening were significantly higher in all the baseline oxygen categories, including patients who breathed in room air, required less than 4 L/min of conventional oxygen supplementation, required 4 L/min or more of conventional oxygen supplementation, and required NIV or HFNC. Among patients enrolled on room air, 26% had plasma levels of 1000 ng/L or higher, which increased to requiring oxygen on the fifth day as compared to 6% of those having plasma antigen levels of less than 1000 ng/L.  

The study findings showed that elevated SARS-CoV-2 N antigen levels were significantly associated with COVID-19 severity and important patient outcomes. The researchers believe that the present study supports the potential role of ongoing SARS-CoV-2 replication in SARS-CoV-2  pathogenesis in COVID-19-hospitalized patients.

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