PROS1 and COVID-19: To determine whether the plasma of COVID-19 patients can block S protein-mediated cell–cell fusion, we incubated the effector cells with COVID-19 convalescent plasma with different titers of NAbs (S2, S3, S21, S23 and S30) at 37 °C for 30 min and then washed the cells prior to co-culturing with the ACE2/293 T target cells.