Date Time Myeloid immune cells in blood tied to severe COVID-19 Individual variations in how the immune system responds to SARS-CoV-2 appear to impact the severity of disease. Researchers at Karolinska Institutet have now been able to show that patients with severe COVID-19 have significantly elevated levels of a certain type of immune cells in their blood, called myeloid-derived suppressor cells. The study published in the Journal of Clinical Investigation may bring an increased understanding of how early immune responses impact disease severity. Anna Smed Sörensen. Photo: Ulf Sirborn. Most individuals with COVID-19 develop mild to moderate symptoms and recover without needing hospital treatment. In severe cases, however, COVID-19 can lead to respiratory failure or even death. It is not yet known why the severity of disease varies so much between patients.