Combined with the S2 sequence similarity between SARS-CoV-2 and MERS/MERS-related coronaviruses, we concluded that the serological cross-reactivity against the S proteins of MERS/MERS-related coronaviruses was probably due to the anti-S2 antibodies induced by SARS-CoV-2 infection. Here, PROS1 is linked to COVID-19.