text: Autoantibodies stabilize neutrophil extracellular traps in COVID 19 The release of neutrophil extracellular traps b NETs b by hyperactive neutrophils is recognized to play an important role in the thromboinflammatory milieu inherent to severe presentations of COVID 19 At the same time a variety of functional autoantibodies have been observed in individuals with severe COVID 19 where they likely contribute to immunopathology Here we aimed to determine the extent to which autoantibodies might target NETs in COVID 19 and if detected to elucidate their potential functions and clinical associations We measured anti NET antibodies in 328 individuals hospitalized with COVID 19 alongside 48 healthy controls We found high anti NET activity in the IgG and IgM fractions of 27 and 60 of patients respectively There was a strong correlation between anti NET IgG and anti NET IgM r 0 4 p 0 0001 Both anti NET IgG and IgM tracked with high levels of circulating NETs impaired oxygenation efficiency and high circulating D dimer Furthermore patients who required mechanical ventilation had a greater burden of anti NET antibodies than did those not requiring oxygen supplementation Levels of anti NET IgG and to a lesser extent anti NET IgM demonstrated an inverse correlation with the efficiency of NET degradation by COVID sera Furthermore purified IgG from COVID sera with high levels of anti NET antibodies impaired the ability of healthy control serum to degrade NETs In summary many individuals hospitalized with COVID 19 have anti NET antibodies which likely impair NET clearance and may potentiate SARS CoV 2 mediated thromboinflammation 