The amount of SARS-CoV-2 antigen measured in the blood of patients hospitalized with COVID-19 is associated with illness severity and other clinical outcomes, according to a new study published in the Annals of Internal Medicine. Following the ACTIV-3 trial of COVID-19 therapeutics in people hospitalized with COVID-19, researchers from the National Institute of Allergy and Infectious Diseases (NIAID), part of the National Institutes of Health, and their collaborators analyzed levels of SARS-COV-2 antigen in blood samples taken from study participants and assessed the association of those levels with disease progression. Higher levels of viral antigen in the blood, which could indicate ongoing SARS-CoV-2 replication, correlated with more severe disease. The authors suggest that SARS-CoV-2 antigen levels hold promise as a biomarker, or a measurable substance, to predict which patients hospitalized with COVID-19 have a higher risk of worse outcomes.