PROS1 and infection: Pre-existing T-cell immunity against protein S was observed in 21 individuals (34%): 8 out of the 12 HCWs with positive serology (67%), 4 out of the 14 convalescents with negative serology at baseline (28%), and 9 out of the 35 infection-naïve (26%), the latter cases attributed to cognate cross-reactivity.