COVID-19: Counting antigens to know its severity

Antigens are particles (protein or polysaccharide type) produced by the virus, here SARS-CoV-2, which, recognized by antibodies or certain immune cells in the body, can trigger an immune reaction. Here, National Institute of Allergy and Infectious Diseases (NIAID/NIH) scientists use transmission electron micrography (TEM, or TEM for Transmission Electron microscopy) to assess the quantity of SARS-CoV-2 virus particles (in yellow on the visual) in infected nasal epithelial cells.

The amount of SARS-CoV- antigens in the blood, directly associated with disease progression

The ACTIV-3 trial: the amount of SARS-CoV- antigens measured in 2,540 blood samples taken from patients hospitalized with COVID-19, all treated with the antiviral remdesivir, is found here to be associated with disease progression. The researchers related, for each participant, their blood antigen levels, length of hospitalization and pulmonary symptoms on day 5. A statistical model was used to determine whether plasma antigen levels were associated with lung function patients and whether they could predict how participants would fare over time. The analysis reveals:

• a strong correlation between higher SARS-CoV-2 antigen levels (≥ 1000 nanograms per litre) and more degraded lung function at baseline;
• a strong correlation between higher SARS-CoV-2 antigen levels (≥ 1000 nanograms per litre) and longer hospital stay;
• these higher levels of SARS-CoV-2 antigens are correlated with certain known risk factors for worsening the disease, such as being a man;
• 3 factors seem to reduce the level of SARS-CoV-2 antigens: the pre-existing presence of antibodies against SARS-CoV-2, exposure to remdesivir before inclusion and a longer duration of hospitalization before inclusion as well;
• infection with the delta variant is associated with higher antigen levels than infection with the previous strains.

These higher levels of viral antigen in the blood could suggest continued replication of the virus and therefore more severe disease, say the researchers who conclude a promising biomarker.

A precision medicine approach could be useful in future clinical trials of antiviral therapy: antigen levels could help determine which patients are most likely to benefit from antiviral therapies.