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The prevailing hypotheses for the persistent symptoms of Long COVID have been narrowed down to immune dysregulation and autoantibodies, widespread organ damage, viral persistence, and fibrinaloid microclots (entrapping numerous inflammatory molecules) together with platelet hyperactivation. Here we demonstrate significantly increased concentrations of von Willebrand factor (VWF), platelet factor 4 (PF4), serum amyloid A (SAA), α-2 antiplasmin (α-2AP), endothelial-leukocyte adhesion molecule 1 (E-selectin), and platelet endothelial cell adhesion molecule (PECAM-1) in the soluble part of the blood. It was noteworthy that the mean level of α-2 antiplasmin exceeded the upper limit of the laboratory reference range in Long COVID patients, and the other 5 were significantly elevated in Long COVID patients as compared to the controls. This is alarming if we take into consideration that a significant amount of the total burden of these inflammatory molecules has previously been shown to be entrapped inside fibrinolysis-resistant microclots (thus decreasing the apparent level of the soluble molecules). We conclude that presence of microclotting, together with relatively high levels of six biomarkers known to be key drivers of endothelial and clotting pathology, points to thrombotic endothelialitis as a key pathological process in Long COVID.

Background: Fibrin(ogen) amyloid microclots and platelet hyperactivation are key pathological findings in patients with acute COVID-19 infection and also in those with Long COVID/Post-Acute Sequelae of COVID-19 (PASC). These pathologies may represent a suitable target for pharmacological treatment of Long COVID. Methods: Here we report on the symptoms displayed by a cohort of 91 South African Long COVID patients at baseline and after a clinician-initiated anticoagulant regime was completed. For laboratory analysis, patients provided a blood sample before and after treatment. Fibrinaloid microclot presence was studied by adding thioflavin T to platelet poor plasma (PPP), whilst platelet hyperactivation was studied using two platelet markers- PAC1 and CD62P (P-selectin). The anticoagulant regime included dual antiplatelet therapy (DAPT- Clopidogrel 75mg + Aspirin 75mg) once a day, and a direct oral anticoagulant (DOAC- Apixaban) 5mg twice a day. A proton pump inhibitor (PPI) pantoprazole 40 mg/day was also prescribed for gastric protection. Each of the treated cases reported their main Long COVID symptoms, and whether their symptoms resolved following treatment or not. Results: In our cohort a most participants did not report any comorbidities before acute COVID-19 infection. Hypertension and dyslipidaemia were the commonest underlying illnesses, whilst the most commonly reported Long COVID symptoms included fatigue, cognitive dysfunction, shortness of breath, and joint and muscle pains. Following completion of treatment, each of the different symptoms resolved in the majority of patients. This was also reflected in the laboratory analysis, where a decrease in the severity of fibrin amyloid microclotting and the degree of platelet pathology was noted. No serious adverse bleeding events were reported. Conclusions: Fibrin amyloid microclots, platelet hyperactivation/ aggregation, and widespread endothelialitis inhibit the transport of oxygen at a capillary/cellular level. This provides a ready explanation for the symptoms of Long COVID. By normalizing the failed clotting physiology and reversal of the endothelialitis, triple anticoagulant therapy represents a promising treatment option that appears to be highly efficacious, and warrants controlled clinical studies. We caution that such a regime must only be followed under expert medical supervision in view of the risk of bleeding.

Long COVID has become a significant global health and economic burden, yet there are currently no established diagnostic tools to identify which patients might benefit from specific treatments. One of the major pathophysiological factors contributing to Long COVID is the presence of hypercoagulability; this results in insoluble amyloid microclots that are resistant to fibrinolysis. Our previous research using fluorescence microscopy has demonstrated a significant amyloid microclot load in Long COVID patients. However, this approach lacked statistical robustness, objectivity, and rapid throughput. In the current study, we have used imaging flow cytometry for the first time to show significantly increased concentration and size of these microclots. We identified notable variations in size and fluorescence between microclots in Long COVID and those of controls even using a 20x objective. By combining cell imaging and the high-event-rate nature of a conventional flow cytometer, imaging flow cytometry can eliminate erroneous results and increase accuracy in gating and analysis beyond what pure quantitative measurements from conventional flow cytometry can provide. Although imaging flow cytometry was used in our study, our results suggest that the signals indicating the presence of microclots should be easily detectable using a conventional flow cytometer. Flow cytometry is a more widely available technique which has been used in pathology laboratories for decades, rendering it a potentially more suitable and accessible method for detecting microclots in individuals suffering from both Long COVID and other conditions with similar pathology, such as myalgic encephalomyelitis.

Earlier variants of SARS-CoV-2 have been associated with plasma hypercoagulability (as judged by thromboelastography) and an extensive formation of fibrin amyloid microclots, which are considered to contribute to the pathology of the coronavirus 2019 disease (COVID-19). The newer Omicron variants appear to be far more transmissible, but less virulent, even when taking immunity acquired from previous infections or vaccination into account. We here show that while the clotting parameters associated with Omicron variants are significantly raised over those of healthy, matched controls, they are only raised to levels significantly lower than those seen with more severe variants such as Beta and Delta. We also observed that individuals infected with Omicron variants manifested less extensive microclot formation in platelet poor plasma compared to those harbouring the more virulent variants. The measurement of clotting effects between the different variants acts as a kind of ‘internal control’ that demonstrates the relationship between the extent of coagulopathies and the virulence of the variant of interest. This adds to the evidence that microclots play an important role in determining the severity of symptoms observed in COVID-19.