PROS1 and COVID-19: Using a highly sensitive ELISA, we detected the presence of immunoreactive S protein in the serum of COVID-19 patients (mean ± SEM: 33.5 ± 8.3 ng/ml), with 18 out of 64 cases having values above the 95th percentile of the control population (>20.9 ng/ml), which comprised samples collected before the pandemic (Figure 4A).