PROS1 and COVID-19: The development of COVID-19 vaccines is mainly based on seven platforms, which can be classified into three modes according to the antigen category.17,18 The first mode is based on the protein produced in vitro, including inactivated vaccines (inactivated SARS-CoV-2), VLP vaccines (virus particles without nucleic acid), and subunit vaccines (S protein or receptor-binding domain (RBD) expressed in vitro).