PROS1 and COVID-19: All subjects (26/26) in the COVID-19 group had a positive test, and we observed that this group showed significantly higher antibody titers of anti-S protein IgG than those in the control group (Fig 2C; P<0.01), which three individuals (3/21, 14.2%) had undetermined anti-S protein IgG serology and two (2/21, 9.5%) were positive, even though showing borderline results (2.0–2.5 O.D. ratio).