PROS1 and COVID-19: Our observation that anti-S1 antibodies from severe COVID-19 patients display a low AI, and are even absent in patients who do not survive, whereas their concentrations increase in convalescent individuals, is consistent with the hypothesis that early defective T-cell help due to T-cell death may compromise IgG antibody production and lower maturation against the S protein of SARS-CoV-2 during the early phase of infection.