Abstract Body (Do not enter title and authors here): Background. Post-COVID syndrome is related to a multisystem disorder that affects in part the cardiovascular system. This disease involves symptoms, and conditions that continue or develop after acute COVID-19. SARS-CoV-2 infection of immune and endothelial cells are associated with NETosis, microthrombosis and endothelial dysfunction that could persist several weeks after acute phase of infection. Damaged endothelial cells can expose the vessel pro-coagulant area leading to platelet and neutrophil clumps. Increased levels of circulating endothelial cells (CECs) have been described as biomarkers for cardiovascular diseases. Therefore, we hypothesize that CECs and microthrombosis are potential biomarkers of vascular dysfunction in Long COVID.

Methods. A cross-sectional study was conducted at the Miami VA long COVID clinic. Long COVID cases and controls were recruited according to WHO definition for long COVID. A total of 23 patients and 7 controls were included in this study. Blood samples were collected in Heparin and Sodium Citrate tubes. Cell immunophenotyping and NETosis markers (MPO) were quantified on a Cytek Aurora spectral flow cytometer system. Microclots (CD62P⁺PAC-1⁺) and platelet response were assessed by flow cytometry and response to Adenosine di-phosphate (ADP), respectively. A ttest was used for statistical analysis. Differences were considered significant when p < 0.05.

Results. The age and gender were similar between cases and controls while their symptom score was significantly different. There was a significant increase in the number of CECs (CD31+CD309+CD45-CD133-) in Long COVID cases. MPO expression in neutrophils (CD11b⁺CD66b⁺CD15⁺) and classical monocytes (CD14⁺CD16^-) was significantly higher in Long COVID. Microclots were significantly elevated, and the platelet aggregation response was dysregulated in Long COVID.

Conclusions. CECs and microthrombosis including NETosis are present in Long COVID and may serve as potential biomarkers or causative mechanisms for vascular dysfunction.