PROS1 and COVID-19: The use of S1 as an immunogen has been proposed for other highly pathogenic coronaviruses, such as MERS-CoV and SARS-CoV, because of its potential high safety profile compared to the use of full-length S. Although full-length S protein can induce the highest immune response, some reports suggested its association with possible side effects in the currently used COVID-19 vaccines [22,23].