Notably, anti-SARS-CoV-2 S protein seroreactivity was detected in all studied COVID-19 naïve samples (n = 9) (Fig. 3B) with more cross-reactivity to S2 subunit as compared to S1 (S2/S1 signal ratio > 1), although one naïve individual (#1) showed a reversed pattern (S2/S1 signal ratio < 1, Fig. 3C). Here, PROS1 is linked to COVID-19.