Moreover, analysis of post-challenge sera showed that SARS-CoV-2-specific antibody responses were directed against the S1 region and ectodomain of the S protein, also with cross-reactive antibody responses against SARS-CoV, but almost no reactivity against MERS and no reactivity against common cold α- or β-coronavirus S proteins, as also shown in human COVID-19 (61). Here, PROS1 is linked to COVID-19.