Moreover, 7.9% (n = 22 out of 280) of the unvaccinated participants with no known history of infection showed antibodies against the SARS-CoV-2 S protein, whereas 20.0% (n = 3 out of 15) of the unvaccinated participants with a known history of infection did not show antibodies against the SARS-CoV-2 S protein. Here, PROS1 is linked to infection.