Platelets from COVID-19 patients are hyperreactive. Platelets were isolated from healthy donors and COVID-19 patients. (A) Representative aggregation traces of washed platelets from ICU COVID-19 patients or healthy donors treated with 2MeSADP (5 nM or 50 nM), thrombin (0.05 U/mL or 0.5 U/mL), or collagen (2 μg/mL or 10 μg/mL) under stirring conditions at 37°C. (B) Quantification of maximal aggregation (N = 7-10 for healthy donors; N = 10-15 for SARS-CoV-2–infected patients). Blue dots indicate ICU patients; red dots indicate non-ICU patients. (C) Washed platelets were allowed to spread on fibrinogen (scale bar, 50 μm; phalloidin stain). (D) After 30 minutes, platelets were fixed and platelet spreading was quantified. (E) Washed platelets were allowed to spread on collagen (scale bar, 50 μm; phalloidin stain). (F) After 30 minutes, platelets were fixed and platelet spreading was quantified. Images are representative for 9 independent experiments. Blue dots indicate ICU patients; red dots indicate non-ICU patients. *P < .05; **P < .01; ***P < .001.