Immunological Biomarkers in Blood to Monitor the Course and Therapeutic Outcomes of COVID-19

Purpose The COVID-19 pandemic has posed a great challenge to the medical community because little is known about its clinical course, therapeutic options, and laboratory monitoring tools for diagnosis, prognosis, and surveillance. This review focuses on immune biomarkers that can be measured in peripheral blood in a clinical laboratory under routine conditions to monitor the innate immune system response in the acute phase, as well as the adaptive immune response established both after infection and vaccination. Methods A PubMed search was performed covering January 2020 to June 2021 to extract biomarkers suitable for monitoring the immune response and outcome of COVID-19 as well as therapeutic interventions, including vaccination. Results To monitor the innate immune response, cytokines such as IL-6 or acute phase reactants such as C-reactive protein (CRP) or procalcitonin (PCT) can be measured on autoanalyzers complemented by automated white blood cell differential counts. The adaptive immune response can be followed by commercially available enzyme-linked immune spot (ELISpot) assays to assess the specific activation of T cells or by monitoring IgA, IgM, and IgG antibodies in serum to follow B cell activation. As antigens of the SARS-CoV-2 virus, spike and nucleocapsid proteins are particularly suitable and allow differentiation between the immune response after infection or vaccination. Conclusions Routine immune monitoring of COVID-19 is feasible in clinical laboratories with commercially available instruments and reagents. Whether biomarkers reflecting the response of the innate and adaptive immune system can be used to make predictions and assist in individualizing therapeutic interventions or vaccination strategies, these strategies need to be determined in appropriate clinical trials. Promising preliminary data are already available based on single-center reports and completed or ongoing vaccination trials.