Thromboinflammation in long COVID—the elusive key to postinfection sequelae?

Long COVID is a public health emergency affecting millions of people worldwide, characterized by heterogeneous symptoms across multiple organ systems. Here, we discuss the current evidence linking thromboinflammation to postacute sequelae of COVID-19. Studies have found persistence of vascular damage with increased circulating markers of endothelial dysfunction, coagulation abnormalities with heightened thrombin generation capacity, and abnormalities in platelet counts in postacute sequelae of COVID-19. Neutrophil phenotype resembles acute COVID-19 with an increase in activation and Neutrophil Extracellular Trap formation. These insights are potentially linked by elevated platelet-neutrophil aggregate formation. This hypercoagulable state in turn can lead to microvascular thrombosis, evidenced by microclots and elevated D-dimer in the circulation as well as perfusion abnormalities in the lungs and brains of patients with long COVID. Also, COVID-19 survivors experience an increased rate of arterial and venous thrombotic events. We discuss 3 important, potentially intertwined hypotheses that might contribute to thromboinflammation in long COVID: lasting structural changes, most prominently endothelial damage, caused during initial infection; a persistent viral reservoir; and immunopathology driven by a misguided immune system. Finally, we outline the necessity for large, well-characterized clinical cohorts and mechanistic studies to clarify the contribution of thromboinflammation to long COVID.