text: In vivo evidence of systemic endothelial vascular dysfunction in COVID 19 Endothelial dysfunction is one of the underlying mechanisms to vascular and cardiac complications in patients with COVID 19 We sought to investigate the systemic vascular endothelial function and its temporal changes in COVID 19 patients from a non invasive approach with reactive hyperemia peripheral arterial tonometry PAT This is a prospective observational case control and blinded study The population was comprised by 3 groups patients investigated during acute COVID 19 group 1 patients investigated during past COVID 19 group 2 and controls 1 1 matched to COVID 19 patients by demographics and cardiovascular risk factors group 3 The natural logarithmic scaled reactive hyperemia index LnRHI a measure of endothelium mediated dilation of peripheral arteries was obtained in all the participants and compared between study groups 144 participants were enrolled 72 COVID 19 patients and 72 matched controls Median time from COVID 19 symptoms to PAT assessment was 9 5 and 101 5 days in groups 1 and 2 respectively LnRHI was significantly lower in group 2 compared to both group 1 and controls 0 53 0 23 group 2 vs 0 72 0 26 group 1 p 0 0043 and 0 79 0 23 in group 3 p 0 0001 In addition within group 1 it was observed a markedly decrease in LnRHI from acute COVID 19 to post infection stage 0 73 0 23 vs 0 42 0 26 p 0 0042 This study suggests a deleterious effect of SARS CoV 2 infection on systemic vascular endothelial function These findings open new venues to investigate the clinical implication and prognostic role of vascular endothelial dysfunction in COVID 19 patients and post COVID syndrome using non invasive techniques 