PROS1 and COVID-19: Sera selected for in-depth analysis were initially assessed for their capacity to bind to WT (Wuhan Hu-1), B.1.1.7, and B.1.351 S proteins by ELISA at time points where binding antibodies were detected (post-vaccinations 1 and 2, convalescent sera additionally evaluated for COVID-19–recovered participants) (Fig. 2D).