Increased platelet activation in critically ill COVID-19 patients. (A-B) The percentage of P-selectin (CD62P) (A) and CD63 (B) surface expression on platelets from SARS-CoV-2⁻ control participants, SARS-CoV-2⁺ asymptomatic subjects, or symptomatic patients presenting mild to severe COVID-19 syndrome. (C) The concentration of TXB₂ in plasma from control subjects or patients with COVID-19 presenting mild to severe syndrome. (D) Quantification of TXB₂, PF4, and PDGF in tracheal aspirates from severe COVID-19 patients under mechanical ventilation (n = 19). The horizontal lines in the box plots represent the median, the box edges represent the interquartile ranges, and the whiskers indicate the minimal and maximal value in each group. *P < .05 between selected groups. ns, nonsignificant.