PROS1 and COVID-19: Significantly higher levels of free protein S were found in the HCs (94.06 ± 8.945%) compared to mild COVID-19 patients (46.35 ± 16.85, p < 0.001), moderate COVID-19 patients (61.55 ± 26.18%, p < 0.01), and severe COVID-19 patients (54.00 ± 20.84, p < 0.001).